〜前に
before (doing)
Verb dictionary form + 前に / Noun + の + 前に
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前に means "before": 寝る前に歯を磨きます, "I brush my teeth before I go to bed".
The verb before 前に is always in the dictionary form, whatever the tense of the sentence. 日本に来る前に勉強しました, "I studied before I came to Japan": the coming hadn't happened yet at the time of studying, so it stays 来る.
After a noun, add の: 食事の前に, "before the meal"; 授業の前に, "before class".
On its own, 前に is "before, previously": 前に一度会いました, "we've met once before". And without に, 前 is simply "in front of": 駅の前, "in front of the station".
Examples
毎晩、寝る前に歯を磨きます。
I brush my teeth every night before bed.
出かける前に電話してください。
Please call before you leave.
日本に来る前に、少し日本語を勉強しました。
Before I came to Japan I studied a little Japanese.
食事の前に手を洗いましょう。
Let's wash our hands before the meal.
授業が始まる前に、トイレに行きます。
I'll go to the toilet before class starts.