This means 'before' or 'ago'. You use it to say something happens earlier than another thing or time.
Verb [dictionary form] + 前にPrimary
Noun + の + 前に
Time Duration + 前に
This means 'before' or 'ago'. You use it to say something happens earlier than another thing or time.
寝る前に、歯を磨きます。
I brush my teeth before going to sleep.
３年前に日本へ来ました。
I came to Japan three years ago.
出かける前に、電気を消すのを忘れないでください。
Before you go out, please don't forget to turn off the lights.
試験の前に、すごく緊張した。
I was really nervous before the exam.
報告書を提出する前に、部長にご確認いただく必要があります。
Before submitting the report, it is necessary to have it reviewed by the department head.
Sometimes you can skip 'に' after '前'. It still means 'before'. Adding 'に' makes it clearer that you are talking about time. If you want to highlight 'before' as a topic, use 'は' or 'も' instead of 'に'.
This is different from 'ato de'. 'Ato de' means 'after'. You use 'mae ni' with a plain verb. For example, 'taberu mae ni' means 'before eating'. You use 'ato de' with a past tense verb. For example, 'tabeta ato de' means 'after eating'.
Don't use this when you want to say "after". The verb before 'mae ni' must always be the plain form. This is true even if the action happened in the past. For example, say "before going" not "before went".
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