Most verb tense agreement problems occur in complex sentences, particularly those employing a perfect tense in the main clause. Not every tense makes temporal sense in a subordinate clause. Sticking to the possible tenses in a subordinate clause will ensure verb tense agreement.
In English, only two tenses are marked in the verb alone: present ("We talk") and past ("We talked"). The other English language tenses are marked by other words, called "auxiliaries," such as have, can, may, do, must and will. For example:
Present perfect: We have talked Past perfect: We had talked Future: We will talk Future perfect: We will have talked
Problems in verb-tense agreement usually occur within complex sentences--sentences with more than one clause. The sentence "When he was a child, he talked to everyone" is complex. The main clause, "he talked to everyone" has one subordinate clause, "When he was a child". In this sentence, the verbs "to be" and "talk" agree because they are both in the past tense. However, if you changed the sentence to "When he is a child, he talked to everyone" the tenses would not agree because in a complex sentence where the main clause is in the past tense, you can only use a present tense subordinate clause to express a universal truth. For example: "Darwin theorized that humans evolve from apes." This is acceptable because "humans evolve from apes" is an expression of universal truth ("all humans"). In complex sentences, the tense of the main clause determines which tenses are possible in the subordinate clause or clauses.
Present Tense Main Clause
With main clauses in the present tense, the following types of subordinate clauses will express different senses of time. Present tense shows an action occurring at the same time: "She is thrilled about the restaurant because she loves the cook." Past tense shows a past action: "We were pleased because we managed to finish cleaning the house." Present perfect tense shows an action continuing from past to present: "The people are happy now because the incompetent politicians have stepped down." Future tense shows an action to come: "They are working hard now because there will be a review soon."
Other Tenses and Possible Subordinate Tenses
When the main clause is in past tense, in the subordinate clause the past tense shows another completed action related in time to the main action; the past perfect shows a previous action; the present tense shows a universal truth or fact. When the main clause is in future tense, in the subordinate clause a present tense shows an action of the same time; a past tense shows a past action; a present perfect tense shows an action that will happen before the main action.
Most verb-tense problems occur in complex sentences where the main clause is in one of the perfect tenses. To avoid this disagreement, remember that when the main clause is in future perfect tense, use the present tense or present perfect tense to clarify the sequence of events; when the main clause is in present perfect tense, use the present perfect tense to show another action related in time to the main action or the past tense to show a past action.