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Which of the following sentences is grammarly correct and why?

When I was a child, I wouldn't go to sleep unless my mom had kissed me goodnight.

or

When I was a child, I wouldn't go to sleep unless my mom would have kissed me goodnight.

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    ...unless my mom kissed me goodnight. The perfect form is "valid", but not so natural. Your second example is completely non-idiomatic to my ear. – FumbleFingers Reinstate Monica Oct 29 '19 at 16:39
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The first is correct and natural, the second is not. A commenter says it is grammatical, but that is not true - it is in fact incorrect use and barely intelligible.

The reason is because it mixes tenses that should agree in that syntax.

When I was a child, I wouldn't go to sleep unless my mom would have kissed me goodnight.

Simply take out the conditional mode to simplify this sentence. "would not go" is modal past simple, so it becomes "did not go". Likewise, "would have" is modal past perfect, that becomes past perfect:

I did not go to sleep unless my mom have kissed me goodnight.

You should be able to see this is a glaring grammar error; part of it is past tense (simple past) and the other is present (present perfect). It's wrong.

Compare:

I did not go to sleep unless my mom had kissed me goodnight.

This is fine, both verbs match in time and therefore the time frame spans the the entire sentence.

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BadZen's answer is entirely correct that the second form is not grammatical because the verb tenses don't agree. There are some other subtleties as well, however:

So, this is wrong:

I wouldn't go to sleep unless my mom would have kissed me goodnight (wrong)

One way to fix the verb tense issue would be to change "kiss" to simple modal to match "would (not) go", which would become:

I wouldn't go to sleep unless my mom would kiss me goodnight. (OK)

This is actually grammatically correct, and there's nothing wrong with this sentence. However, it also doesn't say exactly the same thing as your first sentence:

I wouldn't go to sleep unless my mom had kissed me goodnight. (also OK)

This sentence is also grammatically correct, but technically the use of a modal verb for "kiss" ("would kiss") is talking about volition or potential (that is, your mother was willing to or intended to kiss you), whereas the use of a past perfect form ("had kissed") is talking about actually having been kissed (whether she actually kissed you or not).

In cases like this, the difference is fairly subtle, and many people would use the two sentences interchangeably, because it's generally assumed that if your mother would kiss you, then she also did kiss you, and vice-versa. However, in some other situations the differences between the meanings can be significant.

The point here is that while you do generally need to make verbs agree in terms of past/present/future, number, and so on, when you get into making verbs modal ("would X" / "could X" / "should X"), you don't necessarily want to make all the verbs modal just because one of them is (modality is not something that has to automatically agree). You need to look at each verb in the sentence and decide whether you're actually talking about something actually happening or just the potential of something happening, and then decide from that whether that verb should be modal or not.

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