때 expresses the time when an action or state occurs or its duration. When following a noun 때 is used, and when following a verb ~ ᄅ때 is used. (으) 면 is used when stating a condition about some fact, daily occurence or some repetitive action mentioned later in the sentence, or when supposing and uncertain situation or a situation that has not yet been realized. It means if, when or once in Enlish. I think 파리사 gave you good answer and examples.

There is a huge different Between them (으) 면 has meaning as " IF " For example 한국에 오면 꼭 불고기를 먹을거야" that means as if i come to korea i will eat bulguki for sure but ㄹ 때 has meaning like when "비가 내릴때 따듯한웃을 입으세요" which has meaning as " when the rain comes wear a warm dress

I'm French but from what I understood, I think -(으)면 is used for conditional situation like (If you come you will know -> (만약) 오면 알겠지). On the other hand -(으)ㄹ 때 is used when you explain an event/moment that happened like (when you came you knew -> 왔을 때 알았어요) I'm beginner at Korean, So my explanation might be flawful, could a native tells me if I'm right or not ?) 감사합니다

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@Mojave Right. I think -(으)면 can be used for "when" as it builds an imaginary situation to express both conditional statement as well as expressing what would happen when performing an action.