What is the difference between these two sentences? Simple Past vs Past Perfect Tense?

asfkasfa22
Hi. I am learning English. Please Can i help me?
After they eaten the selfish , they began to feel sick. (past perfect - simple past )

After they ate the selfish , they began to feel sick. ( simple past - simple past)

Which one is right to use? if there are "before" or "after" in sentence. Can i use "simple past - simple past" ?
Updates:
1 mo
sorry.
first sentence : After they had eaten the shellfish , they began to feel sick.
it isn't selfish it is shellfish
What is the difference between these two sentences? Simple Past vs Past Perfect Tense?
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