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If the verb in the main clause is in the future, the verb in the time clause must be in the present tense
Can changes to could in subordinate clauses, when the verb in the main clause is in the past simple tense.
When the verb in the main clause is in the past tense, use a past tense in subordinate clauses.
This rule doesn't apply (1) to verbs within quotations, (2) to facts that are true at all times. We say:
1. She said, 'I am waiting for your answer'
2. He said that London is a great city
If we are speaking of an action just finished, we must use the present perfect instead of the simple past tense. For example, immediately after the clock strikes, we shouldn't say The clock struck, but The clock has struck.
Use the simple present (and not the present continuous) to express a present habitual action.
Use the present continuous to express a habitual action with the word always or with a verb denoting a continuous state: He is always talking in class; He is living in London.