GRAMMAR - Sequence of Verb Tenses

Although the various shades of time and sequence are usually conveyed adequately in informal speech and writing, especially by native speakers and writers, they can create havoc in academic writing and they sometimes are troublesome among students for whom English is a second language. This difficulty is especially evident in complex sentences when there is a difference between the time expressed in an independent clause and the time expressed in a dependent clause. Another difficulty arises with the use of infinitives and participles, modals which also convey a sense of time. We hope the tables below will provide the order necessary to help writers sort out tense sequences.

As long as the main clause's verb is in neither the past nor the past perfect tense, the verb of the subordinate clause can be in any tense that conveys meaning accurately. When the main clause verb is in the past or past perfect, however, the verb in the subordinate clause must be in the past or past perfect. The exception to this rule is when the subordinate clause expresses what is commonly known as a general truth:

- "In the 1950s, English teachers still believed that a background in Latin is essential for an understanding of English."
- "Columbus somehow knew that the world is round."
- "Slaveowners widely understood that literacy among oppressed people is a dangerous thing."

The tables below demonstrate the correct relationship of tenses between clauses where time is of the essence (i.e., within sentences used to convey ideas about actions or conditions that take place over time).


Sequence of Tenses - With Infinitives and Participles:

Like verbs, infinitives and participles are capable of conveying the idea of action in time; therefore, it is important that we observe the appropriate tense sequence when using these modals.

http://grammar.ccc.commnet.edu/grammar/sequence.htm