Dear Teachers,
Thank you very much for your answers.
Now I have some new questions .
Some people say that phrasal verbs are informal, and one-word verbs are formal. Is that right ?. Some Textbooks available in Vietnam, such as Headway, teach how to write a formal or informal letters, but the lessons are not explained clearly . The question is where I can find good lessons about the problem , including Teachers's explaination, vocabulary and lessons for pratice ?