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?

(by kevinwal for everyone)

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