If your native language is Spanish, Portuguese, or French, there is a very specific pronunciation trap waiting for you in English: diphthongs. These are vowel sounds that glide from one position to another inside a single syllable. Your language has some diphthongs too, but English relies on them far more heavily, and the way English diphthongs move is different from what you are used to.
The result? You flatten them. You turn a gliding vowel into a single, steady sound. And native English speakers notice immediately, even if they cannot explain why your accent sounds "off."
This guide will show you exactly what is happening, why your native language causes this habit, and how to fix it with practical exercises.
What Is a Diphthong, Exactly?
A diphthong (from Greek, meaning "two sounds") is a vowel where your tongue and lips move during the sound. It starts at one vowel position and glides toward another. The key word is glide: your mouth does not jump, it slides smoothly.
Think of it this way:
- Monophthong (pure vowel): Your mouth stays still. Like the "ah" in "father."
- Diphthong (gliding vowel): Your mouth moves. Like the "ay" in "say," where it starts open and ends with your tongue rising toward the roof.
English has five major diphthongs, and every single one of them causes problems for Romance language speakers.
Why Romance Speakers Flatten Diphthongs
The root cause is the same across Spanish, Portuguese, and French, even though these languages handle vowels differently from each other.
The Romance Vowel System Is More Stable
Spanish has only five pure vowels: /a/, /e/, /i/, /o/, /u/. They are crisp, clean, and unchanging. When a Spanish speaker sees the English word "day," their brain maps the vowel to the nearest Spanish equivalent: a clean /e/. The glide toward /ɪ/ simply gets deleted.
Portuguese has more vowel variation (nasal vowels, open vs. closed), but the core habit is the same: vowels are targets you hit and hold, not trajectories you follow.
French has a rich vowel inventory (including front rounded vowels like /y/ and /ø/), but French vowels are also predominantly monophthongs. When a French speaker encounters English /oʊ/, they hear their own /o/ and stop there.
Diphthongs Exist in Romance Languages, But They Work Differently
Spanish has diphthongs like "ai" in baile and "au" in causa. But these are typically two clearly separate vowel qualities spoken quickly, not the smooth, gradual glide that English uses. The English /aɪ/ in "my" starts lower and more central than Spanish "ai," and the glide is slower and more deliberate.
This means Romance speakers can produce diphthongs, but they need to retrain the specific starting points, ending points, and speed of the English versions.
Spelling Reinforces the Problem
English spelling is deceptive. The word "go" has two letters, suggesting one vowel. But the vowel is /oʊ/, a diphthong. The word "say" ends in a Y, but the vowel is /eɪ/. Romance speakers, accustomed to more phonetic spelling systems, tend to pronounce what they see: one letter, one sound.
The Five English Diphthongs: A Complete Breakdown
Let us work through each diphthong systematically. For each one, you will learn the starting position, the glide target, why Romance speakers get it wrong, and how to fix it.
1. The /eɪ/ Diphthong (SAY, DAY, MAKE)
How It Works
The /eɪ/ diphthong starts at a mid-front vowel position (like the "e" in "bed," but slightly higher) and glides upward toward /ɪ/ (the vowel in "bit"). Your jaw closes slightly and your tongue rises during the sound.
Why Romance Speakers Flatten It
Spanish, Portuguese, and French all have a pure /e/ vowel. When you encounter English /eɪ/, your ear hears "that's just /e/" and your mouth produces a flat, non-moving vowel. The word "say" becomes "seh," "make" becomes "mehk," and "day" becomes "deh."
The Difference It Makes
Practice Words
2. The /oÊŠ/ Diphthong (GO, BOAT, HOME)
How It Works
The /oÊŠ/ diphthong starts at a mid-back vowel position and glides toward /ÊŠ/ (the vowel in "book"). Your lips start slightly rounded and become more rounded as the sound progresses. Your tongue pulls back and up.
Why Romance Speakers Flatten It
All three Romance languages have a clean /o/ vowel. Spanish "no" has a pure, stable /o/. English "no" has /noÊŠ/, with a clear glide toward a more closed, rounded position. Romance speakers hit the /o/ and stop, missing the second half entirely. "Go" becomes "goh," "home" becomes "hohm."
The Difference It Makes
Practice Words
3. The /aɪ/ Diphthong (MY, TIME, LIKE)
How It Works
The /aɪ/ diphthong has the widest journey of any English diphthong. It starts at a low, open, central position (similar to the "a" in "father") and glides all the way up to /ɪ/ (near the vowel in "bit"). Your jaw closes significantly and your tongue rises dramatically.
Why Romance Speakers Flatten It
This diphthong actually exists in Romance languages (Spanish hay, Portuguese pai, French ail), so speakers can produce it. The problem is quality and duration. The English version starts more centralized (not as far forward as Romance /a/) and the glide takes longer. Romance speakers often produce a quicker, sharper version that sounds clipped to English ears, or they reduce it to a flat /a/ in unstressed positions.
The Difference It Makes
Practice Words
4. The /aÊŠ/ Diphthong (HOW, HOUSE, TOWN)
How It Works
The /aʊ/ diphthong starts at the same low, open, central position as /aɪ/, but instead of gliding up and forward, it glides up and back toward /ʊ/ (the vowel in "book"). Your lips go from open to rounded, and your tongue moves backward and upward.
Why Romance Speakers Flatten It
Portuguese speakers often handle this one better because Portuguese has a similar "au" sound in words like mau and pau. Spanish speakers tend to produce a quicker, tighter version. French speakers often reduce it to something close to /o/ or /a/, missing the rounding entirely. The most common error across all three languages is not rounding the lips enough at the end of the glide.
The Difference It Makes
Practice Words
5. The /ɔɪ/ Diphthong (BOY, COIN, VOICE)
How It Works
The /ɔɪ/ diphthong starts at a mid-back, rounded position (the vowel in "law" for many speakers) and glides forward and upward toward /ɪ/. Your lips start rounded and unround as the sound progresses. Your tongue moves from back to front.
Why Romance Speakers Flatten It
This is often the easiest diphthong for Romance speakers because the "oi" combination exists in all three languages (Spanish hoy, Portuguese noite, French moi). However, the English version has a more open, lower starting position than the Romance equivalents. French speakers in particular tend to start too high (closer to their /wa/ sound in moi), producing something that sounds like "woy" instead of the correct /ɔɪ/.
The Difference It Makes
Practice Words
Practical Techniques to Master the Glide
Understanding the theory is only half the battle. Here are concrete exercises to retrain your mouth.
1. The Mirror Exercise
Stand in front of a mirror and watch your mouth as you produce each diphthong. You should see visible movement:
- /eɪ/: Your jaw should close slightly. You can see your mouth getting narrower.
- /oÊŠ/: Your lips should push forward and become more rounded.
- /aɪ/: Your jaw should close dramatically. This has the biggest visible movement.
- /aÊŠ/: Your lips should go from wide open to a small, rounded shape.
- /ɔɪ/: Your lips should go from rounded to spread (the opposite of /aʊ/).
If your mouth is not moving, you are producing a monophthong. Diphthongs require visible mouth movement. No movement means no glide.
2. The Exaggeration Technique
Deliberately exaggerate the glide until it feels ridiculous. Say the two vowel components separately, then gradually merge them:
- "say" = first say "seh" ... then "ih" ... then blend: "seh-ih" ... then faster: "seɪ"
- "go" = first say "goh" ... then "oo" ... then blend: "goh-oo" ... then faster: "goÊŠ"
- "my" = first say "mah" ... then "ih" ... then blend: "mah-ih" ... then faster: "maɪ"
When you think you are exaggerating too much, you are probably at the level a native speaker uses naturally. Romance speakers consistently underestimate how much movement English diphthongs need.
3. The Recording Test
Record yourself saying these five sentences and compare with a native speaker recording:
- "I say we should take a break." (three /eɪ/ sounds)
- "I need to go home and use the phone." (three /oÊŠ/ sounds)
- "My favorite time is nighttime." (three /aɪ/ sounds)
- "How did you find our house in this town?" (three /aÊŠ/ sounds)
- "The boy made a choice to raise his voice." (three /ɔɪ/ sounds)
Listen specifically for whether you can hear the movement in your vowels. If each vowel sounds like a single, steady tone, you need more glide.
4. The Slow-Motion Drill
Practice each diphthong at half speed. Hold the starting vowel for two full seconds, then slowly glide to the target over another two seconds. This trains your muscle memory for the trajectory. Once the slow version feels natural, gradually increase speed to normal speaking pace.
5. The Contrast Drill
Alternate rapidly between the flat Romance vowel and the correct English diphthong:
- "eh" - "eɪ" - "eh" - "eɪ" (feel the difference between flat and gliding)
- "oh" - "oÊŠ" - "oh" - "oÊŠ" (feel your lips round more on the diphthong)
- "ah" - "aɪ" - "ah" - "aɪ" (feel your jaw close on the diphthong)
This contrast drill teaches your ear to hear the difference and your mouth to feel the difference. Both are necessary for permanent change.
Common Mistakes by Language
Spanish Speakers
- Biggest problem: /eɪ/ and /oʊ/. Spanish has extremely stable /e/ and /o/ vowels, making the glide feel unnatural.
- Hidden trap: The word "no" in English is /noÊŠ/, not /no/. Since it looks and feels like Spanish "no," many speakers never notice they are flattening it.
- Best starting point: Practice /aɪ/ first, since Spanish hay provides a foundation.
Portuguese Speakers
- Biggest problem: /oÊŠ/. Brazilian Portuguese has significantly reduced "ou" to /o/ in most dialects ("ouro" is commonly pronounced "oro").
- Advantage: Portuguese nasal diphthongs (like in mao and poe) mean your mouth is already accustomed to vowel movement.
- Best starting point: Practice /aÊŠ/ first, since Portuguese mau is quite close.
French Speakers
- Biggest problem: /oʊ/ and /ɔɪ/. French /o/ is very pure, and French "oi" /wa/ has a completely different quality than English /ɔɪ/.
- Hidden trap: French speakers sometimes add lip rounding where it does not belong (on /eɪ/ and /aɪ/) because French vowels are generally more rounded.
- Best starting point: Practice /aɪ/ first, since French ail provides a starting reference.
A Quick Reference Chart
| Diphthong | IPA | Starts At | Glides To | Key Word | Romance Trap |
|---|---|---|---|---|---|
| AY | /eɪ/ | Mid-front | /ɪ/ (higher) | say, day | Sounds like "seh, deh" |
| OH | /oÊŠ/ | Mid-back | /ÊŠ/ (rounder) | go, boat | Sounds like "goh, boht" |
| EYE | /aɪ/ | Low-central | /ɪ/ (much higher) | my, time | Too quick or flat "ah" |
| OW | /aÊŠ/ | Low-central | /ÊŠ/ (rounded) | how, house | Missing lip rounding |
| OY | /ɔɪ/ | Mid-back rounded | /ɪ/ (unrounded) | boy, coin | Starting too high or using /wa/ |
Final Thoughts
Mastering English diphthongs is not about learning five new sounds. It is about learning a new way of making sounds: moving instead of staying still. Your Romance language trained your mouth to hit a vowel target and hold it. English wants you to hit a target and then immediately start gliding toward another one.
The good news is that this is a purely mechanical skill. There is nothing conceptually difficult about diphthongs. Your mouth can already make all the necessary positions. You just need to practice connecting those positions with a smooth glide instead of treating them as separate stops.
Start with the diphthong that your specific language struggles with most (see the section above for your language). Practice it in isolation, then in words, then in sentences. Use the mirror, the recording test, and the exaggeration technique. Within a few weeks of focused practice, the glide will start feeling natural, and native speakers will notice the difference immediately.