如果 (rúguǒ) means "if". It starts the condition, and 就 (jiù) often comes in the second part to mean "then". In Chinese the condition almost always comes first.
如果 + condition, (subject) + 就 + result.
如果 can go before or after the subject of the first part: 如果你累了 and 你如果累了 are both fine. 就 goes before the verb in the second part. It isn't always needed, but it makes the link between the two parts clear.
If the second part has its own subject, 就 comes after the subject, not before it. Say 我就去, not 就我去. If the second part has no subject, 就 starts it.
You can add 的话 at the end of the condition: 如果……的话. It means the same and is very common in speech. With 的话, you can even leave out 如果.
In everyday speech, people often drop 如果 and keep only 就.
如果 works for real and imagined situations alike, so 如果我是你 is the way to say "if I were you". To say what will happen as a result, use 就会 + verb.
In English the "if" part can come second: "I'll go if you go". In Chinese, put the condition first: 如果你去，我就去.
如果 only means "if", never "when". For something you know will happen, use a time word such as 以后 instead.
Practise this grammar point on paper: 19 exercises using only HSK 3 words, with an answer key on a separate page. Free to print and copy for your class.
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