就【jiù】

就 marks the second half of a condition as its result: "if this, then that". It sits after the subject of the result clause, right before the verb.

The conjunction in front sets the kind of condition – 如果 a plain "if", 只要 "as long as", 既然 "since", 一旦 "once". 就 stays the same after all of them; each has its own point for the details.

With no "if" word

In speech the first clause often has no conjunction at all. 就 is then the only thing that tells you the first clause is a condition.

Drop 就 from 你不去我不去 and it reads as two flat statements side by side.

那就 in replies

In conversation the condition is often the other person's sentence. 那就, or 那么就 in writing, picks it up: "in that case".

就 or 才

就 says the condition is enough for the result. 才 says the condition is the only way to get it, and pairs with 只有, not 只要.

Where 就 goes

就 is an adverb, so it never stands before the subject. When the result clause has its own subject, the subject comes first.

• × 要是他不来，就我们自己做 → 要是他不来，我们就自己做
• × 只要你同意，就我去办 → 只要你同意，我就去办