[55a.] In dependent clauses and infinitives, the tense is to be considered in relation to the time expressed in the principal verb.

[b.] When narration in the past tense is interrupted for reference to a preceding occurrence, the past perfect tense is used.

[c.] General statements equally true in the past and in the present are usually expressed in the present tense.

[d.] The subjunctive mode of the verb to be is used to express a condition contrary to fact, or a wish.

[e.] Use the correct auxiliary. Make sure that the tense, mode, or aspect of successive verbs is not altered without reason.