What Sequence of Tense Is. How and Why It Matters
The problems of sequence of tense are most seriously felt when dealing with the past tenses and the subjunctive. This lesson will give the general contours of the problem involved and provide some rules of thumb, using examples involving the use of the subjunctive. Although this article is not about the subjunctive, the link above will lead learners to a book in which all the problems of the subjunctive, including the logic of sequence of tense, are treated in detail, with exercises and answer keys.
The reason that the problem of sequence of tense shows up so much when the subjunctive is involved is because the subjunctive is a verb form that appears in a subordinated clause -- that is, in a clause that is introduced by a verb in a previous clause. Let's look at an example: Yo quiero que tú vengas a mi casa esta noche (I want you to come to my house tonight). The main verb is quiero -- a verb in the present indicative. The time frame is established by the main verb. The subjunctive is necessary because that verb is of a type that "triggers" the need for it if there is a subordinated clause -- and sure enough, the verb venir must be in the subjunctive (vengas), conjugated to agree with the subject of the subordinated clause - tú. Notice that it is in the present subjunctive.
If the same sentence is changed so that the main verb is past, the need for the subjunctive mood remains, but the tense must be changed to be in harmony with the past time frame. Yo quería que tú vinieras a mi casa esa noche (I wanted you to come to my house that night).
Although there is more to the story, if learners are alert to the time frame as established by the main verb in a sentence, most of the problems will be reduced to the logical application of the tenses -- a distinct problem from the ones posed by the need for the subjunctive or the indicative.