Julinka

Hello! Who can correct my sentences?!? Thank you a lot! Julinka

Hello my darling,
Sherlock Holmes will not leave you before you will have improved your English.
I am happy to see you again. Thanks a lot for your welcome !

Romance-Writer

Your sentences are correct but they don't roll off the tongue very well.

Hello my darling, (or just hello darling)
Sherlock Homes won't leave you before you've improved your english. I'm happy to see you again and thanks for welcoming me.

will not = won't
you will = you'll
you will have = you've

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Jean Charest t'es un gros criss de cave et t'as clairement identifié comme étant un baril de poulet Kentucky dans l'vinaigre. Mon plus grand désir est de te voir se faire rentrer en prison pour jouer à "échappe le savon" avec des gros sales tatoués

Adieu Jean Charest, tu ne nous manqueras surtout pas

Jean Charest démissionné le 5 septembre 2012

Julinka

Thank you! But you will have = You've ?!? I thought "you've" means "you have got ..."

Romance-Writer

It's just the way we communicate and it can be both depending on the context.

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Jean Charest t'es un gros criss de cave et t'as clairement identifié comme étant un baril de poulet Kentucky dans l'vinaigre. Mon plus grand désir est de te voir se faire rentrer en prison pour jouer à "échappe le savon" avec des gros sales tatoués

Adieu Jean Charest, tu ne nous manqueras surtout pas

Jean Charest démissionné le 5 septembre 2012

MikeL

English (unlike French and many other languages) doesn't use the future tense after conjunctions like "before", "when", "as soon as", etc.
When you arrive, you will find many people there.
Likewise the present perfect is used instead of the future perfect:
As soon as you have finished, we will go out.
So it is a question of tense usage or idiom rather than style.

Romance-Writer

Sadly everything you just explained still more or less goes over my head. That's because none of it's ever taught at any level in the majority of anglophone schools. I'm currently at the university master's level and until I started seriously studying other languages, I had never heard of those grammar constructs.

So I'm not really in a position to comment on your explanation of the subject. Because all I know is that we would write and/or say.

Therefore...

When you arrive, you will find many people there.
Would be "When you arrive, you'll find many people there."
As soon as you have finished, we will go out.
Would be "As soon as you've finished, we'll go out. "

Regardless of nationality, I'm sure that if you asked 100 anglophones what the "present perfect" or "future perfect" is, at least 90% of them wouldn't be able to guess; not even if under threat of death.

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Jean Charest t'es un gros criss de cave et t'as clairement identifié comme étant un baril de poulet Kentucky dans l'vinaigre. Mon plus grand désir est de te voir se faire rentrer en prison pour jouer à "échappe le savon" avec des gros sales tatoués

Adieu Jean Charest, tu ne nous manqueras surtout pas

Jean Charest démissionné le 5 septembre 2012

MikeL

But the advantage of learning the terminology is that you have a way of explaining different structures efficiently, and don't need to resort to generalisations like "it's just what we say". And the point about the examples above is not to do with "you'll" instead of "you will" - a simple contraction - but with why we say "when you arrive" instead of "when you'll arrive".