Learn Hardcore Japanese: ryouri wo tukuru mae ni yasai wo araimasu.

When you want to say “before doing X,” you must attach 前に to the dictionary form of the verb. You cannot say 作った前に. By contrast, if you want “after doing X,” you use the た-form plus 後で (e.g. 作った後で).

Can we say 料理を作るの前に using の?

No. When attaching 前に directly to a verb, you do not use の. The correct form is 作る前に, not 作るの前に. You only use の when turning a noun phrase into a modifier, but verbs already link to 前に on their own.

What role does the particle に play in 前に?

In time expressions, に marks a point in time. Here, 前に (“before”) functions like an adverbial phrase meaning “by that time” or “preceding that event.” So 前に = “at a time before ...”

Why do both 料理 and 野菜 take the particle を?

The particle を marks the direct object of an action. In 料理を作る, 料理 (food/dish) is what you make. In 野菜を洗います, 野菜 (vegetables) is what you wash.

Could the time clause 料理を作る前に appear at the end of the sentence?

Normally, time clauses like ～前に come before the main clause in Japanese. Placing it at the end (e.g. 野菜を洗います、料理を作る前に) sounds disjointed or unnatural. It’s best to keep 料理を作る前に at the beginning or directly before 野菜を洗います.

What is the politeness level of 洗います?

洗います is the polite (ます-form) present/future tense of 洗う. This sentence is at a polite level, suitable for everyday conversation with co-workers, teachers, or people you don’t know well.

How would you say the opposite sequence, “I wash the vegetables and then cook the food”?

You can reverse the order by using てから or た後で:

野菜を洗ってから料理を作ります。
野菜を洗った後で料理を作ります。