急が___、電車に遅れますよ。
Q. Which form completes the warning 'if you don't hurry...'?
Translation
If you don't hurry, you'll miss the train.
Explanation
〜ないと + negative consequence is the everyday warning conditional: 急がないと遅れます. なくて states a reason, and ないので would assert the not-hurrying as fact.