(NOTE.—Should and would follow the rules for shall and will.)
+61. Tenses for the Completed Action.+
1. To represent an action as completed at the present time, the past participle is used with have (hast, has). This forms the present perfect tense: [I have finished].
2. To represent an action as completed in past time, the past participle is combined with had (hadst). This forms the past perfect, or pluperfect, tense: [I had finished].
3. To represent action that will be completed in future time, shall have or will have is combined with the past participle. This forms the future perfect tense: [I shall have finished].
+62. Sequence of Tenses.+—It is, in general, true that the tense of a subordinate clause changes when the tense of the main verb changes. This is known as the Law of the Sequence (or following) of Tenses: [I know he means well. I knew he meant well].
The verb in the main clause and the verb in the subordinate clause are not necessarily in the same tense.
[I think he is there. I thought he was there.
I think he was there. I thought he had been there.
I think he will be there. I thought he would be there.]
In general, the principle may be laid down that in a complex sentence the tense for both principal and subordinate clauses is that which the sense requires.
General truths and present facts should be expressed in the present tense, whatever the tense of the principal verb: [He believed that truth is unchangeable. Who did you say is president of your society?].