The train leaves at 3 o’clock tomorrow.
In this sentence we are talking about an action which will happen in the future according to some schedule, in such situations the Present Simple is used in the English language.
Another good example of this rule is:
My algebra lesson starts at 8 in the morning tomorrow, so set the alarm, please, as I don’t want to be late for it!
If I see John next week, I will speak to him about this matter.
The sentence expresses the action which may happen in the future (next week) on condition that I see John, nevertheless, the Present Simple is used in such sentences in English.
Let’s remake our subordinate clause of condition to the subordinate clause of time:
When I see John next week, I will speak to him about this matter.
As far as you can see in the time clause still the Present Simple is needed.
The Smiths are visiting us tonight.
The action is going to happen in the near future, but the Present Continuous is chosen in English to talk about such events.
If you want to prepare well, check our Advanced Grammar in use webinar.