Strictly speaking, the modals are not tensed. However, they developed historically from tensed verbs. Each group below shows a "historical present / past" pair.

can / could
shall / should
will / would
may / might
must - not paired

In modern English, any of these can be used in a single clause sentence, but when it occurs in just one of two or more clauses in the same sentence, it needs to be matched correctly with the tense in the other clause.

Typically, when the accompanying clause is an "if" clause, the historical present tense forms go with present tense verbs, and historical past tense forms go with past tense verbs. However, "could" and "might", meaning "may" ("it is possible"), can be used with present tense verbs (as well as with past tense verbs). "should" has lost most of its force as a past tense and is better thought of as a present tense form in modern English, meaning "it is advisable". ("shall" is the least used of the modals shown above.) The result of all these historical changes is that in modern English we have far more choices in sentences with a present point of view than in those with a past point of view.

With this in mind we have:

If she has [present] time, she [can, could, (shall), should, will, may, might, must] go to the party. [not "would']

If she had [past] time, she [could, would, might] go to the party. [not "should"]

For reported speech we have the pairs:

He says that he [can, could, (shall), should, will, would, may, might, must] go to the party.

He said that he [could, could, (shall), should, would, would, might, might, must] go to the party.

Note how "can", "will", and "may" are restricted to the present, and the corresponding forms "could", "would", and "might" are used to report "can" or "could", "will" or "would", or "may" or "might" in the past.

CJ

All correct. Look at the list again. With "if" statements, "could" is possible for both present and past in the "if" clause. But remember that "could" is a substitute for "may" in the case of the first two in the present. If you want to express that she is able to go (to the party), you will have to use "can".

"go" by itself would indicate "leave the area", "leave the premises". This would not be a situation in which one would need to consider whether one has time, so the first and third sentences are slightly anomalous. However, they are not anomalous at all if they are intended to contain ellipsis of the destination already mentioned earlier in the conversation.

Do you think she could go to the concert?
Yes. If she has time, she [could, may, might, can] go.

CJ

Good question. All modals are available for a statement with a single clause. So when you report what someone says in the present tense (he says, she says, ...), you don't change the words they used. "would" is available in a direct quote, so it is available in a present-tense report.

Jonathan says, "What would you do if your car were stolen?"
Henry says, "I would notify the police."

Note: Henry leaves off "if my car were stolen". It is unnecessary to repeat it.

Reporting Henry's statement we can have either of the following, depending on which tense we want to report in.

If we say it right after hearing it, we will probably use the present tense.

"Henry says that he would notify the police". We can add "if his car were stolen" if we want.

If time has passed, we may prefer to use the past tense.

"Henry said that he would notify the police". We can add "if his car were stolen" if we want.

The optional additions I mention above make more clear the reason that "would" is chosen. It's chosen because it was a "conditional" idea in the original statement.

An alternate way of stating is to use the so-called "third conditional" form:

"Henry said that he would have notified the police (if his car had been stolen)."

Perhaps is even better than , because (without the "if" clause) can also mean: "Henry said that he was going to notify the police", and can't.

CJ

Thanks for the clear explanation.

Are these correct? If not, why? For these ones, how does 'would' work? How come it doesn't need to be conditional?
1. That would be cool if you can do that. (Why isn't 'would' 'will'?)
2. It would be nice of you if you can do that. (Why isn't 'would' 'will'?)

What do these mean?
3. That will be cool if you can do that.
4. It will be nice of you if you can do that.