received your letters

The second one is better. Especially if you're still learning.

Otherwise, you would write "I learned English for...."

Same comment. The first one suggests continuity.

Thank you. Then do you agree we can say?:

-They've repaired the roads for 5 years/for months.

It depends on the rest of the theme. (context)

examples

They've been repairing the roads for months, but they're just as bad as ever.
(the work is presumably continuing)
They've repaired the roads for five years, but they've stopped all work now.
(refers to an action in the past)

I'd say that the oddity comes with the use of the verb 'learn', especially with the present perfect (PP). We often use the PP for things that have occurred over a period of time and show no indication of ending/ show continuity.

He has/I've studied English for more than fifty years.

She has/I've lived here for ten years.

They/I have taught ... for/since ... .

Thank you~

You have refrained from using HAVE REPAIRED with FOR MONTHS but FOR 5 YEARS; aren't these two time expressions equal in grammatical status?

I don't know quite what you meant by this, and I don't want to guess. Would you care to re-phrase it?

I've got a feeling you did not completely get the meaning of my last answer.

Sorry~ Let me rephrase it:

Do you not think they are just the same?

They've repaired the roads for five years.
They've repaired the roads for months.

(not in meaning, but in grammar)

Yes, they are the same.

As they are the same that leads to WB's question;

You have refrained from using HAVE REPAIRED with FOR MONTHS but FOR 5 YEARS; aren't these two time expressions equal in grammatical status?

JTT and Mctag have seen HAVE BEEN REPAIRING/REPAIRED without saying they are wrong; clearly they as native speakers think the two are correct. It was a surprise to me.

A long time ago, my teacher, a non-native speaker, told me: REPAIR in grammar is known as an accomplishment verb; if you use an accomplishment verb in the present perfect tense, you must not use it with a time expression like FOR MONTHS/FOR 5 MONTHS, so

-They've been repairing the roads for months
is correct but
-They've repaired the roads for months
is definitely wrong.

Do you think this non-native speaker is wrong?

I've purposefully left those uses alone for a reason, WB, except to remind McTag of your question to him.

I've been trying, by means of examples, to lead you into a more
colloquial turn of phrase.
The above sentence "They've repaired the roads for months" is not ungrammatical, but in most instances/ contexts that I can think of, a different phrase would be used. As JTT often advises, context is all.
If you are describing a continual, but intermittent process WHICH IS STILL GOING ON, you would say "They've been repairing the road for months."

(btw that would apply whether or not the repairs were intermittent.)
My examples above are an attempt to demonstrate that.

Thank you~