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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.
Will changes to would in subordinate causes, when the verb in the main clause is in a past tense.
If we are speaking of the result of a past action rather than of the action we must use the present perfect tense. When somebody says, I have seen Panthenon, he or she is not thinking so much of the past act of seeing it, as the present result of that past action.
Use the past tense after the phrase as if or as though. He talks as if he knew everything, means He talks as he would talk if he knew everything.
Use the subjective were with the verb to be after as if: He acts as if he were a rich man.
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.