I had John return the video for me.
In this sentence, why do we use return and not returns or returned?
This sentence is an example of an indirect command or request. Such sentences are considered by some grammarians to be an example of the subjunctive, which in this case appears as the verb itself. (This is what I was taught in grade school.)
Others (see comments) argue that this need not be classified as subjunctive, but rather as a use of the infinitive:
… we would use him in the same construction: I had him return the video for me. This means there is no subordinate clause with omitted that (*I had [that] he return the video), but rather a simple object (him) + infinitive (return). The fact that you can’t add that supports the same conclusion. — Cerberus.
Whatever the explanation, this usage is not dependent on the tense of the main verb of the sentence, which can be future, past or present.
Other examples of indirect commands:
His mother demanded he be home by 12. (Past)
The doctor has patients provide a detailed medical history. (Present)
Henry will insist we come early to the party. (Future)