Sequence of Tenses
The rules governing verb tenses are dictated by logic; an action in the future obviously cannot happen before an action in the past. In writing, it’s a matter of looking at your clauses and sentences, and determining when each action is happening. The past must come before the present, and the present before the future, etc. Pay particular attention to the verb sequence when you have a dependent clause before the independent clause, or a result clause before the if clause.
When the independent clause is in the past tense, the dependent clause may be written in the past or possibly the present (see Exceptions), but not the future.
The cat was bathing because his feet are dirty.
Because the tense of the independent clause is in the past (was bathing), the verb in the second clause (are) is in the wrong tense.
The cat was bathing because his feet were dirty.
The cat is bathing because his feet are dirty.
Exceptions: There are two exceptions to this rule:
for cases involving universal knowledge
Even the early doctors knew that the washing of hands prevents infection.
when using a modal which has no past tense form
Could you please help me move this bookshelf?
Of course, this doesn’t mean that the actual verbs have to be in chronological order, just the actions. We can put the dependent clause at the beginning of the sentence.
Athena will continue to learn English when she gets to the States.
It’s alright to have the future tense (will continue) before the present tense (gets) because the temporal conjunction when shows that the second action actually happens first.
Watch out for conditional clauses, too.
We’ll go for a walk if the weather held.
The future tense of the result clause is too distant from the past tense of the if clause.
We’ll go for a walk if the weather holds.