Why would you do that?

Question

When my friend say

I'm gonna wear weird costume,

could I say

Why would you do that?

in future meaning?

When my friend say

I took your panties yesterday,

could I say

Why would you do that?

too? in past situation?

Answer 1

Would is a Modal Auxiliary Verb, a group of auxiliary verbs with peculiar properties. One of them is that they don't have any tense, so the normal mode of an English auxiliary verb is not open to them. Normally, the first auxiliary in the [verb chain] inflects for tense:

• The model [has been being photographed] for hours now.

The auxiliaries do, be, have, and get all inflect for present and past tense, like all verbs in English:

• He is not here today. ~ He was not here yesterday.
• He doesn't need any money now. ~ He didn't need any money then.
• He hasn't realized it yet. ~ He hadn't realized it yet.
• He gets married tomorrow. ~ He got married yesterday.

But modal auxiliaries don't inflect for any tense, present or past:

• He must leave tomorrow, but not *He musted leave yesterday
• He should leave now, but not *He shoulded leave yesterday

Since there are only two tenses in English and neither is marked on modals, they're considered to be tenseless. There is no "future tense" in English, no matter what you've been told in school, since all modals can refer to any time -- will is another modal auxiliary and behaves like the rest of them).

So you can say "Why would you do that?" about any event someone has done, or is doing right now, or is considering doing. It's all "would-be", and that's timeless.

Answer 2

Why would you do that?

is less about tenses and more about expressing a somewhat negative surprise or amazement, sometimes enhanced by adding ever:

Why would you ever do that?

It is in fact part of a conditional sentence in which the if clause is omitted but implied. The implied part could be for example:

• If you were in your right mind, ...
• If you were my real friend... etc.

Answer 3

In this case, "would" is not indicating an action in the future.

This is an example of the second conditional. The second conditional is used for unlikely events. The events you describe may very well be unlikely, depending on the context.

This kind of question might be better asked at the English Language Learners Stackexchange.

Answer 4

The answer depends on what exactly you are trying to express. The key thing to have in mind it the difference between expressing something as 'actual' and expressing something as potential, doubtful or at least not certain. Latin and ancient Greek verbs had special modes, subjunctive and optative (known for these two languages as 'moods' along with the - to us - more common 'indicative' mood), to express things as not actual in some way. English does not have these. It has, instead a modal version of the auxiliary, would, or sometimes could.

So in the present case, where an intention to wear weird costume, the addressee uses the modal form of the auxiliary to express a kind of disbelief: that dressing like this is so inappropriate, odd or unlike the person that 'would' is used as opposed to 'why are you going to do that?'.

In the second case, the same response, using 'would' is still standard English. It expresses a sort of shocked or appalled disbelief. Or they might have said 'how could you have done that?', which brings with it a sense of rebuke, as in "I thought I knew you better than that."

This answer is not in terms of the more conventional notion of 'first', 'second' etc conditionals. It is simpler to understand in terms of what is going on.