Taking the exact words from an original source is called quoting. You should quote material when you believe the way the original author expresses an idea is the most effective means of communicating the point you want to make. If you want to borrow an idea from an author, but do not need his or her exact words, you should try paraphrasing instead of quoting.
How often should I quote? Quote as infrequently as possible. You never want your essay to become a series of connected quotations, because that leaves little room for your own ideas. Most of the time, paraphrasing and summarizing your sources is sufficient (but remember that you still have to cite them!). If you think it’s important to quote something, an excellent rule of thumb is that for every line you quote, you should have at least two lines analyzing it.
