Hi,everyone. I saw a grammar book saying that in the following case we can use pressent/future tense in a relative clause:

Let's see who finishes/will finish first.

But that book doesn't explain clearly in what condition can this rule be applied to. I guess maybe it's because the imperatives. So I've found several similar cases to let everyone see if they could all use pressent/future tense in the relative clauses:

Let me know how you are from time to time. (Can we say "Let me know how you will be from time to time." ?)

Let's see who gets there first. (Can we say "Let's see who will get there first." ?)

Come back in a week and tell me what you think then. (Can I say "Come back in a week and tell me what you will think then."?)

Make sure you come back soon. (Can I say "Make sure you will come back soon." ?)

If it is not because imperatives, then why can the first sentence use pressent/future tense in its relative clause? Please enlighten me with this, thank you!
Only one s in present!!! Emotion: smile
You can't use will with the stative verbs in this construction: [Let's see / Let me know / Tell me] plus an indirect question. I would not call these relative clauses.

to finish first - an action.
how you are - a state - not an action.
get there first - same as arrive first - an action.
you think - an internal activity; having an opinion - not an action.

Thus, with the indirect question in brackets { }, ungrammatical choices prefixed with asterisk *:
[Let's see / Let me know / Tell me] {who [finishes / will finish / gets there / will get there] first}.
[Let's see / Let me know / Tell me] {how [you are / *you will be / what you think / *what you will think]}
Your example with make sure does not belong to the pattern above. There's no indirect question.
Make sure (that) you come back soon.
A make sure that pattern is not followed by a future (will).
You can save yourself a lot of headaches by using the present tense for all of these. Even in cases where will is possible, it's almost always the inferior choice.
Thank you, I understand now!