When to Quote and When to Paraphrase
Generally, you should quote an author directly when he or she has expressed an idea in such a powerful way that putting the passage in your own words would weaken it. You should paraphrase passages in which the information, not the actual language, is what will strengthen your assignment. It is always good to support your ideas and opinions with information from experts, but remember not to quote or paraphrase too much. After all, your mentor wants to read your work, not someone else's. An unwritten rule is that quoted and paraphrased material should comprise no more than 20 percent of a paper.
While it is true that you do not need to cite sources for everyday phrases such as the early bird always gets the worm or no news is good news, in most other cases it is good to err on the side of being too careful. It is much better to do a few extra minutes of work than it is to be accused of committing plagiarism.