先 (xiān) marks the action that happens first, and 然后 (ránhòu) introduces what comes next. Together they put two or more actions in order, like "first... then" in English.
Subject + 先 + action 1, 然后 + action 2.
先 is an adverb, so it goes after the subject and directly before the verb. 然后 starts the second part of the sentence. If both actions are done by the same person, you don't need to say the subject again.
The most common mistake is to follow English "First, I..." and put 先 before the subject: 先我去银行 is wrong. Say 我先去银行.
然后 works the other way round. It joins the two parts, so if the second action has a new subject, that subject comes after 然后: 然后你说, not 你然后说.
You will often hear 再 before the second verb, either after 然后 or on its own: 先……再…… means the same as 先……然后. For a third step, use 最后 (finally).
Practise this grammar point on paper: 16 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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