~지만 means "but." ~(으)면 means "if" (and sometimes "when") 일을 하자면... will be "[I] work but..." so I might say something like 좋아하는 일을 하지만 힘들어요. (I do work that I like but it's hard/tiring). 일을 하면... will be "If [I] work..." I might say something like 좋아하는 일을 하면 행복해요. (If I do work that I like I am happy.) I could even combine the two: 좋아하는 일을 하면 힘들지만 행복해요. (If I do work that I like, it's difficult/tiring but I'm happy). some more examples: 미나가 말이 많은 사람이지만... Mina is a talkative person but... ...화가 나면 조용해져요. if/when she's angry she gets quiet. 커피 맛있지만... Coffee is delicious but... ...커피를 마시면 잠이 안 와요. if/when I drink coffee I can't sleep.