Qfyilyi

We use the present simple for scheduled events in the near future such as " I have an appointment next week". Why , then, is "My friend will pick you up at 4:00" the way we say it. Why isn't "My friend picks you up at 4:00" correct? Isn't this also a scheduled event in the near future?

The key here is to understand the difference between the form of an inflected verb, and the state of events it actually refers to. So, we have in English grammar present tense, which normally refers to present time, but not always.

Huddleston and Pullum call use of the present tense to refer to future time scenarios the "futurate". They comment as follows:

The futurate construction is subject to severe pragmatic constraints:
the clause must involve something that can be assumed to be known
already in the present. The three most common uses involve cyclic
events in nature, scheduled events, and conditionals. (Cambridge Grammar of the English Language, p.132)

Your example about a scheduled event in the future fits in well with this general statement.

We can use the present for scheduled events in the near future. However, we don't use the present at the time we are scheduling the events.

The sentence

My friend will pick you up at 4:00.

sounds to me like I am informing the listener of the arrangement. In this situation I would use will pick you up rather picks you up, I think because I don't consider the event to have been scheduled until the listener is informed.

On the other hand, if the listener already knows about the schedule, and we are just confirming the arrangements, I would use picks you up.

And in situations like the concert starts at 7:00, I would use the present tense regardless of whether the listener knows already, because this event is scheduled even before the listener is informed.

you can use the simple present to express fixed arrangments whether in future or present:
Your exam starts at 09.00.
Also, To express future time, after some conjunctions: after, when, before, as soon as, until:
He'll give it to you when you come next Saturday.

You can study more in the following link:

http://www.ef.com/english-resources/english-grammar/simple-present-tense/

But, when you use "will" for future tense, it could be an instant decision. "My friend picks you up at 4:00" could be correct if it was an arrangement. You can recognise it in the context.

have is not an action that occurs at a specific time, it describes a relationship between two entities, in this case the person and the appointment. This relationship exists continuously during some period of time, and we use the present tense if we're currently in that period. So you would say "I have that book" at any time while you own it; you only use the future tense if the relationship doesn't exist yet:

I will have that book after I buy it.

But when an action occurs at a specific time, we use the tense appropriate for that time. A verb like to go can be used this way.

I went to my appointment yesterday.

or

I will go to the appointment next week.

However, there's lots of flexibility. You can say either:

I will go on vacation in March.

or

I am going on vacation in March.

Using the present tense essentially expresses the idea that your future plans are part of your current state of being -- you are now a person who is going to go on vacation in March.

You can also use the present tense for repeated actions, when the time period of the repetitions surrounds the current time:

Mary picks up her kids from school at 3pm.

You would only need to use past or future tense if you wanted to refer to a specific occasion of picking up her kids, rather than the general action.

absolutely, present tenses can be used instead of will+infinitive to refer to the future in subordinated clauses:

I'll write to her when I have time.

I'll have a good time whether I win or lose.

and for the main verb that does not have a future form:

Phone me when you arrive.

Make sure you come back soon.

I reference my examples to:

Practical English Usage, 4th edition - Michael Swan's guide to problems in English