Some English words carry a letter that you never say out loud. The clearest example is the silent B before T. In debt, doubt, and subtle, the B is written but never pronounced. Say "det," "dout," and "sutt-le."
This is not random. Knowing the small group of words that follow this pattern means you will never again add an awkward /b/ where native speakers hear silence.
The Rule
When B comes directly before T inside these historic words, the B is completely silent. The words came into English from Latin and French, and scholars later inserted a B to show the Latin root (debitum gave debt, dubitare gave doubt, subtilis gave subtle). The spelling changed; the pronunciation never did.
The silence carries into every word built from these roots: doubt, doubtful, doubtless, undoubtedly; debt, debtor, indebted; subtle, subtly, subtlety. The B stays silent in all of them.
Practice Words
The Important Exception
The B is silent only in this fixed etymological group. When BT sits across a real prefix boundary, you DO pronounce the B. In obtain /əbˈteɪn/, obtuse, subtitle, subtotal, and subtract, the prefixes ob- and sub- keep their B fully voiced. So the safe approach is to memorize the silent set (debt, doubt, subtle and their families) and pronounce the B everywhere else.
Quick Tip
Group them by sound, not spelling: debt rhymes with bet, doubt rhymes with out, and subtle rhymes with cuttle (as in cuttlefish). If your version rhymes cleanly with those, your B is correctly silent.