테면 is short for 터이면. 터 is a dependent noun which means a plan or will. 면 is a conditional ending particle which means "if". So, the structure can be explained as follows. (너는) 내일 갈 터이다. = You have a plan to go tomorrow. (You will go tomorrow.) 내일 갈 터이면 = 내일 갈 테면 = If you have a plan to go tomorrow (If you go tomorrow) As I said, 테면 is simply the abbreviation form of 터이면 and we don't consider it slang.

no. it might be comprehensive under the situataions. but, no problem at all to talk to friends.

thaiholic~ Thank you very much for your responses on both of my questions! Your responses are very helpful to me ^o^ Would this be considered as slang because it's "wanna try" or can it also be translated as "want to try"?

I hope my advice is helpful to you. As you know, one word(Korean) is used with lots of meaning. ‘면’ = if ‘볼테면’ = if you wanna try(the speakers’ willness is include) But we don’t use it only ‘볼테면’, it need to plus other Verb. Ex. ‘해볼테면’ = if you wanna try to do, (cf. do=하다) ‘맛볼테면’ = if you wanna try to taste, (cf. taste=맛보다) But, ‘볼 테면’ = if you wanna try to see, (cf.보다) ‘볼 테지’, without other words(Subject/object), I am not sure what you mean. Ex, 볼 테면, 볼 테지 = if you wanna try to see, just see it! But this sentence’s nuance is like to complain or grumble. If you wanna see it, just see it without any saying, huh! When = 하면, 할 때(up to the situation) When you finish your homework, you can go out. 네가 숙제를 다하면(다 할 때) / 끝내면(끝낼 때), 너는 나갈 수 있어.