Simple and perfect tenses
I have a big trouble (or problem, what is correctly in this case?) with choosing between simple and perfect tenses.
When I have a conversation by skype, if my companion does not understand my verbal phrase, I duplicate it writing. How to formulate a request to wait while I’ll do it (write) Is it correctly to say:
«Wait a minute, now I'll write you a message».

If I just wrote a message, and the companion has not seen it yet, how to say correctly:
«I just wrote you a message»
or
«I’ve just written you a message»

Is the using of verb tenses in all this text correct?

And, if you want, call me, My skype is olga_brabets

Question

Simple and perfect tenses
I have a big trouble (or problem, what is correctly in this case?) with choosing between simple and perfect tenses.
When I have a conversation by skype, if my companion  does not understand my verbal phrase, I duplicate it writing. How to formulate a request to wait while I’ll do it (write) Is it correctly to say:
«Wait a minute, now I'll write you a message».

If I just wrote a message, and the companion has not seen it yet, how to say correctly:
«I just wrote you a message»
or
«I’ve just written you a message»

Is the using of verb tenses in all this text correct?

And, if you want, call me, My skype is olga_brabets

Answer

Hi Olga,

The basic difference is that past simple describes the past and present perfect describes the present (by using a past event). With perfect tenses, you're using an earlier event to describe a later point in time. Remember that past tenses never describe the present.

For your phrases...

"I have a big problem..." or "I always have trouble..." (Problem is countable; trouble is uncountable.)

"Wait a minute, I'll write you a message." ("Now" is unnecessary.)

"I've just written you a message." is what you need in your situation.  Remember, a past action to describe the present: "have written".