conditional tenses

please can anyone help me analyse the tenses in this conditional sentence:

"If we did not stay up so late, we would not be so tired."

What's the tense of "did not stay up" here? It's not past perfect, because that would be:

"If we had not stayed up so late, we would not be so tired."

The implication seems to be that we're in the habit of staying up late, both in the past and also the present. But there is also the 'hypothetical' implication that if we were not to stay up late (in the future), we would not be so tired.

Thanks for any help anyone can offer.

The the verb phrase did not stay up late is past tense (technically only the word did actually has any tense). Notice that we use the term tense to refer to a particular form (inflection) of a verb. Technically, this is different from talking about which time it refers to.

In terms of time reference, as opposed to tense, we cannot tell from this sentence whether it refers to the past, to the present or to the future. The most natural reading is that the sentence is referring to a hypothetical present:

• We always do badly in our Maths test on Monday morning. If we didn't stay up so late we wouldn't be so tired.

Here didn't stay up so late refers to staying up late in general in the same way that I don't smoke refers to time in general. We can think of it as present time, but it is really referring to "always": the past, present and future together. Here the past simple tense is used because the not-staying-up-late situation is presented as a hypothetical one. It certainly is not a current actual situation.

However, the context could give us a reading that was clearly only about the future:

• A: I don't want to go there in the morning. We'll be really tired after playing cards all night.
• B: We could go in the morning. If we didn't stay up so late we, wouldn't be so tired.

Here we B is referring to future time, not the present. Again the past simple is used not to refer to past time, but because the situation is being presented as a hypothetical one.

However, it may equally be the case that the past tense is being used purely to refer to past time:

• When I was a boy we used to go to bed at around 4 on Sunday night, and we'd always fail our Monday morning maths test because we were so tired. Occasionally though we would go through a patch of being sensible and go to bed on time. If we didn't stay up so late we wouldn't be so tired and we'd often be top of the class.

Here the use of past tense is not because of any "hypothetical situation". It does not represent "modal remoteness". It is used simply to refer to past time.

Conclusion

The string didn't stay up is a past simple construction. We know this because of the past tense of the auxiliary verb did. With regards to time reference, we do not know whether this is a hypothetical conditional with backshifted verb forms or normal conditional which uses tense in the same manner as any other sentence. We can only tell this from the context of the sentence. The sentence could be referring to past time, general time, present time or future time.

The verb "did" is a past tense form, though its meaning has nothing to with past time, but with modality by virtue of indicating a degree of remoteness or non-factuality as is typically found in conditional clauses like this. This remoteness is referred to as 'modality' and hence the verb is usefully called a 'modal preterite'. "Stay" is of course the plain (base) form because it follows an auxiliary verb ("did").