How do you distinguish combination and permutation question?
An example of a combination question:
Example: How many different committees of 4 students can be chosen from a group of 15?
Answer: There are possible combinations of 4 students from a set of 15.
There are 1365 different committees.
An example of a permutation question:
Example: How many ways can 4 students from a group of 15 be lined up for a photograph?
Answer: There are 15P4 possible permutations of 4 students from a group of 15. These are different lineups.
How to know in which case to use $nCm$ and in which $nPm$?