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After the verbs can, must, may, shall, and will, use the infinitive without to , and not the third person of the present.
Don't use must and ought to as past tenses. To express a past duty (which wasn't done) use the perfect infinitive without to after ought to or should, or expressions such as had to, was obliged to.
In indirect speech use must and ought to as past tenses: He said he must do it.
If you begin with a verb referring to past time, keep the verb forms in the past. The same rule applies to tenses throughout a composition.
Express an improbable condition by the past tense and use the conditional in the main clause. This use of the past tense doesn't indicate a time but a degree of probability.