прежде чем vs до того как

prezhde chem vs do togo kak — before vs before that

What it really means

Прежде чем introduces a subordinate clause and often highlights the action that must happen first. До того как also means “before,” more explicitly presenting one event as a time boundary. Both take a finite clause with как/чем and require a comma before the subordinate clause when it follows the main clause.

Examples in natural speech

Подумай, прежде чем отвечать.
Think before answering.
Прежде чем уйти, закрой окно.
Before leaving, close the window.
До того как начался фильм, мы купили попкорн.
Before the film began, we bought popcorn.
Он позвонил до того, как приехал.
He called before he arrived.
Проверь адрес, прежде чем отправлять письмо.
Check the address before sending the letter.

Common mistakes

Do not omit the comma around a fronted subordinate clause. «Прежде чем» can take an infinitive («прежде чем уйти»), while «до того как» normally introduces a finite clause; do not confuse до того with до этого.

Memory tip

Прежде чем is a warning before an action; до того как draws a timeline boundary.

Frequently asked questions

Are they interchangeable?

Often yes, but прежде чем more readily gives advice or a prerequisite, while до того как sounds more chronological.

Can прежде чем take a noun?

No; use «перед» plus instrumental for a noun: «перед поездкой», or «до» plus genitive.

Why is как sometimes preceded by a comma?

In «до того, как» many editors use a comma because the clause boundary is explicit; punctuation also depends on placement and intonation.

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