How do you know when to use sin, cos and tan?

First draw the triangle and label all the information you have as well as the unknown angle or side length. There should be at least one angle labelled with some informtion - use this angle to label the opposite, adjacent and hypotenuse sides of the triangle. If the opposite and hypotenuse are being used, use sine for the calculations. If the adjacent and hypotenuse are used, use cosine. If the opposite and adjacent are being used, use tangent. You can remember this using the acronym SOH CAH TOA.