In English, the past tense suffix is pronounced as [-t] in verbs like slapped, missed, and faked. However, it is pronounced as [-d] in verbs like danced, killed, smeared, gazed, banned, and nabbed. What is the reason for this?

It has to do with voicing assimilation (voiced sounds vs, unvoiced sounds at the end of the word stem.) before the suffix is added.

Explanation:

Ex ...

'P' is a voiced consonant (with your hand in front of your mouth, you feel a puff of air on your hand), so it's phonologically easier (more natural) to say a '-t' sound immediately afterward.

'N,' in contrast has no puff of air, so it's a voiceless sound.