I try to learn english. Now I am doing some exercises, but not shure my answers right or not. Can anybody help me?

To start with, let me correct some of your mistakes:
I'd say: I am trying to learn English. ..., but I am not sure if my answers are right.
What exercises are you doing? How can I help you?

wow! thanks! I will write now some of them, ok?

You are welcome

For example, I am learning now new topic for me "Attachments" (to email):
Please complete the attached forms, and return them to me ... 3 June
Answers: A by, B until, C before, D to.
I think, the right answers are B and C.
What do you think?

In any case "by 3 June" is correct - that means "not later than 3 June"
I'd also say "until" and "before" are also possible. But never "to".
What about the task of the exercise? How many answers can be given?

Thanks! The next, please:
Here is the itinery for Sri Lanka. Please A/C that I have included everything you want in it.
A check, B agree, C control, D confirm.
In the exercise is written: "in each gap there are two possible answers."

"check" and "confirm"
Not "control" since it means "regulate" or "rule". It used to mean "check" before but this meaning has become archaic in modern English.
"confirm" means "to say that something is correct or true"

Thank you! Can I ask you else?
"There is one mistake in each of these sentences. Correct it":
Attached are the two questionnaries - please return them me by 24 September.
I think, "the" is a mistake here. Don't need "the" in this sentence?

Certainly you may! ("Can I ask you else?" sounds rather Russian, you'd better say "May I ask you some more questions? May I ask anything (something) else?)
Attached are the two questionnaries - please return them to me by 24 September. "the" is correct here

"Thanks for sending me the report - I let you know what I think." ("The" is missing? correct would be ... for the sending..? )

"Thanks for sending me the report - I will let you know what I think."
No "the" before "sending"

"Here's the report - hope you like it." (I cannot find a mistake in this sentance)

"Here's the report - I hope you like it." The subject is necessary here (in contrast to our Russian construction "Надеюсь, Вам понравится"), though I'd even say "I hope you'll like it". In colloquial spoken language (e.g. in a dialogue) you can omit "I" but I'd use it in written language.
Judging by the "mistakes", I assume the book the exercises are taken from is Russian. Am I right?

I don't know the name of this book exactly, because I have only copies of several pages. I guess, it should be a german book (I am living in Frankfurt now). The neme of book is approximately "Email" or "Learn write email". Not shure.
Please one sentense else, ok?
"I'm returning your original document with my comments inserted with red." (Ich hab keine Ahnung, was für einen Fehler gibt es hier.)

"I'm returning your original document with my comments inserted in red."
Mind your spelling: "sure", "sentence", "German", "name"
"One more sentence" (not "one sentence else")

Some more sentances. They are really last.
" (I) Hope you'll be capable to open the document this time."
"Please check the attached document careful(ly) and let me know if you have any questions."
There are my answers in the brackets.

"Hope you'll be capable of opening the document this time." It seems here that the authors prefer the colloquial variant, so "I" can be omitted here. So you can also omit it in the previous sentence using "you'll like it". But "capable of doing sth." is the correct expression.
"Please check the attached document carefully and let me know if you have any questions."
answers in brackets (without "the")

Thank you very much!
Are you not tired at such late time?
Can you suggest me any prefer possibilities for learning english online? I prefer online, because I am a student and have very cheap internet.

Read books in English - this is the best possibility to learn languages.
There are also some useful links which you can use:
http://www.lbt-languages.de/ (cklick on free learning aids on the left hand side)
http://www.anglik.net/ - variuos links for English as a foreign language
http://www.cjr.org/tools/lc/
http://www.englisch-hilfen.de/
http://www.englishskills.com/
http://esl.about.com/
http://www.better-english.com/
http://www.bbc.co.uk/worldservice/learningenglish/
Is it enough?
I am a late bird and never go to bed before midnight. But now I think I'll fall asleep soon.
What do you study?