之前 / 之后【zhīqián / zhīhòu】

之前 means "before" and 之后 "after". Both go after the event or time they are measured from, and the whole phrase comes before the main clause.

After the event, not before it

English puts "before" and "after" in front. Here they follow what they refer to.

• × 之前吃饭，要洗手 → 吃饭之前，要洗手
• × 他之后下班去了超市 → 他下班之后去了超市

Standing alone

At the head of a sentence, with nothing in front, 之前 and 之后 point back at a moment already in the conversation: "before that", "after that".

之后 or 以后

以前 and 以后 do the same job after an event, and are more spoken. The difference is the bare word: 以后 alone means "in the future", from now; 之后 alone means "after that", from a point in a story.