They’d fall for that trick VS They fell for that trick
SCRIPT
Lynette: I can still see your heads. The pool’s not deep enough.

(the boys' heads pop up over the edge of the hole. Once they realise they’re getting a pool they start digging again with no complaints. Lynette smiles, she knew they’d fall for that trick)

QUESTION
1. What if I change the sentence a bit?
From "they'd (would(?) fall for that trick"

To "they fell for that trick" or " they had fallen for that trick"

Is this possible? Which one is best suitable expression in this context?

2. Someone fall for the trick
- Is this common in use?

Thank you so much!

Question

They’d fall for that trick VS They fell for that trick
SCRIPT
Lynette: I can still see your heads. The pool’s not deep enough.

(the boys' heads pop up over the edge of the hole. Once they realise they’re getting a pool they start digging again with no complaints. Lynette smiles, she knew they’d fall for that trick)

QUESTION
1. What if I change the sentence a bit?
From "they'd (would(?) fall for that trick"

To "they fell for that trick" or " they had fallen for that trick"

Is this possible? Which one is best suitable expression in this context?

2. Someone fall for the trick 
-> Is this common in use?

Thank you so much!

Answer

1) You would not be able to change the sentences without changing the meaning a little bit.

"She knew they'd fall for the trick" means she knew BEFORE she told them that the boys would dig more if she mentioned a pool. This sentence is giving you a glimpse into Lynette's thoughts prior to her statement as a first person point of view.

By changing it to, "They fell for that trick" you are changing the point of view to a third person narrator and making a general statement that the boys were conned into digging more.

1.5) You are right. They'd = they would

2) "Someone fall for the trick" would not be common. As it is with the verb "fall", it sounds like a command. And commanding someone to fall for a trick defeats the purpose of it being a trick. But the phrase "~~fell for the trick" is common.

Examples:
She fell for my trick.
Don't fall for his tricks!
I know they always lie, but I keep falling for their tricks.