Tense is the way grammar expresses time through different verb forms. At least, that is what we normally think. Tense in English, however, is sometimes not directly related to time in the real world. The Present Simple can be used to talk about other times:
The future: We leave at six tomorrow morning, (a plan)
The present: I take two eggs, beat them, then mix in the flour, (a demonstration)
The past: So - just as we agreed - I ring her. I do my best to be nice to her.
And what thanks do I get for it? Nothing! Just told never to ring again! (a person telling a story)
All time: I love my wife, (a statement which we hope will always be true!)
So, take care when you give rules to students about the tenses. Remember that it can be better to say nothing, rather than give a rule which is sometimes true and sometimes not.