watasi ha yoru neru mae ni ha wo migakimasu.

夜寝る前に breaks down as:

• 夜 (yoru) “night”
• 寝る (neru) “to sleep” (dictionary/plain form)
• 前 (mae) “before”
• に (ni) a particle that tells you “at that time” or “before that event”

So 前に means “before (something happens).” The に is required whenever you turn 前 into a time‐adverbial phrase (“before X”)—you cannot say just 寝る前 in this kind of sentence when you want to specify “at that point in time.” Together, 夜寝る前に means “before going to sleep at night.”

Japanese clausal modifiers (like indicating time with “before sleeping”) attach to verbs in their plain (dictionary) form:

• 寝る前に = “before sleeping.”

The main verb of the sentence, 磨きます, is in polite form because the entire sentence is being stated politely. So:

• Use plain form when a verb modifies another part of the sentence (e.g. expresses “when,” “after,” “before,” “because,” etc.).
• Use -ます (or another main‐clause ending) for the sentence’s final verb when you want a polite tone.

Japanese word order is relatively flexible, but modifiers typically come before the words they modify. Here:

• 寝る (to sleep) modifies 前 (before) → 寝る前.
• 夜 (night) modifies the entire phrase “before sleeping” → 夜寝る前.
• Finally, に turns it into a time adverb → 夜寝る前に.

You cannot say 前に夜寝る, because that would literally read “before, at night go to sleep.” And 寝る前の夜に would awkwardly mean “on the night before sleeping,” which is redundant (every night is before you sleep). The natural flow is Time-when → (subevent) → marker.

Yes. If you want to say “I brush my teeth before I sleep in the morning,” which doesn’t make sense physically but grammatically might be:

• 朝寝る前に歯を磨きます
  But usually people would say:
• 寝る前に歯を磨きます (“I brush my teeth before going to sleep”)
  or specify another time, e.g.
• 朝ごはん前に歯を磨きます (“I brush my teeth before breakfast”).
  You just swap 夜 with whatever time frame you need.