from the comments to the OQ:

• You'd have to be very upset with her not to answer the phone when she calls/called you.

 

If you had to further explain the choice of "calls" or "called" to an inquisitive English learner, who is quite interested in syntax/grammar, what would you say? Learners often want to see the "why" of things, and that's a challenge for me.

 

I mean, I've seen people say things like "would requires the conditional or even the subjunctive mood (thus "called"), while when requires the indicative mood (thus "calls"), but I honestly don't know if that explains much of anything.

A reasonable request. You're quite right, naming names doesn't explain anything. Especially if the names refer to grammatical phenomena in other languages, like Latin.

In the example sentence, the would part of you'd refers to a judgement of the speaker on the degree of upset that the negative infinitive clause not to answer ... you shows. Put another way,

• If you don't answer the phone when she calls, (then) you'd have to be very upset.
• If you didn't answer the phone when she called, (then) you'd have to be very upset.

Normally, we'd match up the tenses in the if clause and the then clause, as above. But there is no tense for answer in the original example; it's in an infinitive clause, and they don't have any tense. The speaker left no clue in the if clause for the tense of the then clause.

There are a number of possibilities for the context:

1. You always hang up when she calls you.
2. You sometimes (or at least once) have hung up when she called you.
3. You have not yet hung up on her.
4. She has never called you.

In (1), the speaker would probly use calls, because it's a current context. In (2), either one might be used, depending on how recently the hangup occurred, how frequently she had called before, and what has happened since. In (3) and (4), the situation is entirely imaginary, so called seems most likely, since this is irrealis (which is often confused with "subjunctive"), and past tense forms are used for that, as in

• I wish they were coming tomorrow.

which implies that they aren't coming tomorrow.

from the comments to the OQ:

• You'd have to be very upset with her not to answer the phone when she calls/called you.

If you had to further explain the choice of "calls" or "called" to an inquisitive English learner, who is quite interested in syntax/grammar, what would you say? Learners often want to see the "why" of things, and that's a challenge for me.

I mean, I've seen people say things like "would requires the conditional or even the subjunctive mood (thus "called"), while when requires the indicative mood (thus "calls"), but I honestly don't know if that explains much of anything.

A reasonable request. You're quite right, naming names doesn't explain anything. Especially if the names refer to grammatical phenomena in other languages, like Latin.

In the example sentence, the would part of you'd refers to a judgement of the speaker on the degree of upset that the negative infinitive clause not to answer ... you shows. Put another way,

• If you don't answer the phone when she calls, (then) you'd have to be very upset.
• If you didn't answer the phone when she called, (then) you'd have to be very upset.

Normally, we'd match up the tenses in the if clause and the then clause, as above. But there is no tense for answer in the original example; it's in an infinitive clause, and they don't have any tense. The speaker left no clue in the if clause for the tense of the then clause.

There are a number of possibilities for the context:

1. You always hang up when she calls you.
2. You sometimes (or at least once) have hung up when she called you.
3. You have not yet hung up on her.
4. She has never called you.

In (1), the speaker would probly use calls, because it's a current context. In (2), either one might be used, depending on how recently the hangup occurred, how frequently she had called before, and what has happened since. In (3) and (4), the situation is entirely imaginary, so called seems most likely, since this is irrealis (which is often confused with "subjunctive"), and past tense forms are used for that, as in

• I wish they were coming tomorrow.

which implies that they aren't coming tomorrow.