For 1, I'd write "Limit your time on line. Otherwise, one day you'll find yourself completely alone with no friends." Note the space between "on" and "line". This is a case where the adjective is one word, "online", but the noun is a two-word phrase.
For 2, you were correct. "Create a good impression" is not natural.
For 3, the hyphen might be possible. It's impossible to say without a complete sentence. This could be a similar case to "on line" versus "online". "IT-breakthrough" isn't a common adjective, but that's how I would spell it if I were using it as one. As a noun, it should be "IT breakthrough".
Finally, you should write "Thanks in advance."
Student or Learner