Daren't is the short form of dare not. You use it when you are too afraid, or too careful, to do something: “I daren't tell her.” It is mainly British. American speakers more often say “I don't dare” or “I wouldn't dare.” The n't is the same little piece you hear in don't, can't, and won't. Write it with the apostrophe in the middle: daren't. In most everyday talk, daren't is for now. For the past, English usually wants didn't dare, not daren't. Though actually, you may still meet older writing where daren't is used more widely; for a learner, “I daren't” (present) and “I didn't dare” (past) is the safe pair.