Strictly speaking, in English, only two tenses are marked in the verb alone, present as in "he sings" and past as in "he sang." Our other tenses, as many as thirty of them, are marked by other words called auxiliaries. Understanding the six basic tenses allows one to recreate much of the reality of time in his writing. The six are:
simple present: they walk present perfect: they have walked
simple past: they walked past perfect: they had walked
future: they will walk future perfect: they will have walked
Problems in sequencing tenses usually occur with the perfect tenses, all of which are formed by adding an auxiliary or auxiliaries to the past participle, the third principal part.
ring, rang, rung, walk, walked, walked
The most common auxiliaries are forms of "be," "can," "do," "may," "must," "ought," "shall," "will," "has," "have," "had", and are the forms we shall use in this most basic discussion.
The present perfect consists of a past participle (the third principal part) with "has" or "have." It designates action which began in the past but which continues into the present or the effect of which still continues.
1. Betty taught for ten years. (simple past).
2. Betty has taught for ten years. (present perfect).
The implication in (1) is that Betty has retired; in (2), that she is teaching.
1. John did his homework. He can go to the movies.
2. If John has done his homework, he can go to the movies.
Infinitives, too, have perfect tense forms when combined with "have," and sometimes problems arise when infinitives are used with verbs such as "hope," "plan," "expect," and "intend," all of which usually point to the future (I wanted to go to the movie. Janet meant to see the doctor.) The perfect tense sets up a sequence by marking the action which began and usually was completed before the action in the main verb.
1. I am happy to have participated in this campaign!
2. John had hoped to have won the trophy.
Thus the action of the main verb points back in time; the action of the perfect infinitive has been completed. (Notice present, present perfect sequence in the comment.)
The past perfect tense designates action in the past just as simple past does, but the action of the past perfect is action completed in the past before another action.
1. John raised vegetables and later sold them. (past)
2. John sold vegetables that he had raised. (past perfect)
The vegetables were raised before they were sold.
1. Renee washed the car when George arrived. (simple past)
2. Renee had washed the car when George arrived. (past perfect)
Note the difference in meaning between these two sentences.
In sentences expressing condition and result, the past perfect tense is used in the part that states the condition.
The future perfect tense designates action that will have been completed at a specified time in the future.
1. Saturday I will finish my housework. (simple future)
2. By Saturday noon, I will have finished my housework. (future perfect)
1. Judy saved thirty dollars. (past)
2. Judy will save thirty dollars. (future)
3. Judy has saved thirty dollars. (present perfect)
4. Judy had saved thrity dollars by the end of last month. (past perfect)
5. Judy will have saved thirty dollars by the end of this month. (future perfect)