Past tense

I'm confused about the formation of the past tense. In my Simplified manual, some words seem take a 'd' (e.g. received), others a 't' (e.g. placed).

I realise if I stop and think about it that the received ends in more of a 'd' sound, and placed in a 't' sound, but stopping and thinking takes time, which affects speeds.

Is there a rule?