In which condition we use combination and in which condition permutation.

This depends on the statement of question. If in the statement of question you finds out that repetition of objects are not allowed and order matters then we use Permutation. e.g. Find the number of ways that a party of seven persons can arrange themselves in a row of seven chairs. See in this question repetition is no allowed because whenever a person is chosen for a particular seat r then he cannot be chosen again and also order matters in the arrangements of chairs so we use permutation here. If in the question repetition of samples are not allowed and order does not matters then we use combination. A student is to answer eight out of ten questions on an exam. Find the number m of ways that the student can choose the eight questions See in this question repetition is not allowed that is when you choose one question then you cannot choose it again and also order does not matters(i.e either he solved Q1 first or Q2 first) so you use combination in this question.