What's difference between "se" and "îşi"?
"se" is for himself, "îşi" for something else of his, even a part of himself. For example, "El își vede cămașa în oglindă." = "He sees his shirt in the mirror." or "El își vede fața în oglindă." = "He sees his face in the mirror."
How do I know if it's "in A mirror" or "in THE mirror" when there's a preposition that requires the indefinit ending of the noun?
You can use the indefinite article after all preposition except "CU". Of course, if you then have an adjective, you need to use the definite article -> El se vede în oglinda mare.
Why is he looks at himself in the mirror wrong?