当…的时候 / 当…时【dāng … de shíhou / dāng … shí】

当 opens a "when" clause that …的时候 closes. The whole frame comes before the main clause, which says what happened at that moment.

…的时候 can do the same job alone; adding 当 makes the sentence more careful and a little more formal. 当…时 is the written version, common in articles and instructions.

当 needs its closing half

当 can't introduce the clause alone. Without 的时候 or 时 it doesn't mean "when".

• × 当我小，我住在北京 → 当我小的时候，我住在北京
• × 当他来，我们开始吃饭 → 当他来的时候，我们开始吃饭

The when-clause comes first

English lets "when" come at the end. Here the time frame goes before the main clause.

• × 我很高兴当你来的时候 → 当你来的时候，我很高兴