"You will fail unless you try."
Translation:Bạn sẽ thất bại trừ khi bạn cố gắng.
There are two meanings of try. "Try to do something" and "try doing something". The first implies that failure is possible, "We tried to win, but we failed". The second is just where you do something to see what the experience is like, as in "I tried the pasta and it was delicious" or "I've tried listening to classical music, but I don't like it". There's no sense that we can fail at the latter examples.
I'm pretty sure "cố gắng" means the first meaning and "thử" means the second meaning. Therefore, "cố gắng" is the only correct option here.
awesome explanation Alastair Hill. I learned vietnamese through immersion so a lot of words I learned through context. Sometimes I forget to think of the English word for something when I'm learning the Vietnamese word, I just connect the Vietnamese word to the appropriate action, emotion, etc. Really because my brain can't multitask lol. The thought never crossed my mind that these are the same word in English. Good observation and good explanation