Would 1

Here's another extremely common use of "would" that has nothing to do with the past tense in conversations.

Jack: What happened to Matt's arm? It's all bruised up.

Jane: Oh, you won't believe this. He paid his cousin $5 to punch his arm 5 times. That's how he got his bruises.

Jack: What! That is stupid! Why would you do that ? See switching subjects.
(It's the same as saying why would anybody do that? or why would he do that?)


"Would" here can also be used when talking about the future


Tim: I don't want to give him money, coz I'm afraid he'll yell at me.

Sara: Why would he yell at you for giving him money?


Explanation:
"would" can be used when you think that what somebody did or does or will do does not make any sense or illogical.