I am writing literature review and need a suggestion regarding how much I can copy from previous study and how much I can write my own. If I m writing by own what are the important points to be focused?
You should start by defining the purpose of your review then think about its scope according to how much good quality relevant literature is available. You should also divide your review into themes which are broader than your specific topic. You should not over-read or under-read: aim for about 2 relevant studies found for every one you include. Also major on the majors - write more about the most relevant literature and less about others. Don't allow your review to degenerate into an annotated bibliography. You need to include theory in your review - it shouldn't be entirely empirical - there are different ways of doing this. Learn how to write good paragraphs- about the right length with an introductory topic sentence, a main body which develops the topic and a conclusion. Finally, focus your themes together at the end of your review on your research questions, re establishing your gap in the literature.
You can use related information from other previous studies but my advice is to always paraphrase, compare studies, highlight differences/similarities/gaps, identify question(s), discuss, and conclude.
I think you should include the focus, background, and results of the study with reference to available written literature on that topic. After providing citation , discuss in your own words in few lines. Do the same after giving each citation. Literature review is mainly the references of other works aligning with your own study. You may quote other writers and elaborate in your own words