生词卡的混乱!!!!!!!!!!!

Hello!!!! i saw this 就得 a while ago and put on a flash card, the english definition was "(if)...then must". (which makes no sense what so ever, and i cant seem to find this in a dictionary or translation site. I dont remember where i saw it, it could be that i used this definition for the word wrongly, or there supposed to be some periods in there somwhere can someone explain and use it in a sentence??????      THANKS!!!!!!