[rewboss]

Language teachers use "tense" as MikeNewYork does here. Linguists use the term in a narrow sense.

For example:

I have seen this film before.

A teacher would say that "have seen" is the present perfect tense. A linguist would say that it is the perfect aspect of the present tense.

I prefer not to talk about the "future tense" in English. Linguistically there is no such thing, but also we tend to use present tenses to talk about the future.

Here's an example of a dialogue (from a real-life TV show):

Sarah: What are you doing?
Doctor: I'm making a jamming detector.
Sarah: What happens if whatever's doing the jamming jams the jamming detector?

Notice how "jams" is a present tense but actually refers to a possible future event. (A common mistake among non-native speakers is to use "will" here, because they are taught that "will" is the future tense.)

We have many different ways of referring to future events:

If it is an event that is 100% definite (often because it is a regular event) which does not require any special preparation, we use the simple present: "The train arrives at 6.34 tomorrow."

If it is part of a condition that must be met (i.e., an "if" or "when" clause), we use the simple present: "If it rains tomorrow..."

If we are making a prediction, we use a modal verb like "will" (if we are confident), or "may", "might", "could" or "should" (if we are not so confident): "It might rain tomorrow."

If, at the moment of speaking, we are making a decision, we use "will" or "shall": "What happens if the whatever's doing the jamming jams the jamming detector?" -- "Er... I shall build in a protective circuit."

If the decision has already been made, but preparations have not yet been made (or preparations are unnecessary), we use "going to": "I'm going to buy a new car." "I'm going to phone her tonight."

If the decision has already been made and all the preparations are complete, we use the present progressive: "I am flying to Corfu next week -- I have the tickets right here."

The last two uses are sometimes confusing, and very often we can use either. But sometimes there are differences:

"I'm going to take Jill out for a meal at her favourite restaurant." (Jill doesn't know about this yet.)

"I'm taking Jill out for a meal at her favourite restaurant." (I've already invited Jill, and she said "Yes".)