Learn Hardcore Japanese: watasi ha yoru neru mae ni ha wo migakimasu. - I brush my teeth before going to bed at night.

In Japanese, は marks the topic of the sentence (“as for…”), not the grammatical subject. By saying 私は, you’re introducing “me” as the context for everything that follows—“As for me, I brush my teeth…” If you used が (the subject marker), you’d be emphasizing or introducing 私 in a different way (“It is I who brush my teeth”), which sounds more like you’re contrasting or answering a question about who does the action. For a simple habitual statement, は is the natural choice.

What is the function of 夜寝る前に and why is に attached to 前?

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.”

Why is 寝る in the plain/dictionary form before 前に, but 磨きます in the polite -ます form?

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.

Why is を used with 歯?

を marks the direct object of a transitive verb—what the action is done to. Brushing teeth is an action performed on your teeth, so you say 歯を磨きます (“brush teeth”). Without を, Japanese would not know which noun is being acted upon by 磨きます.

Could you drop 私は and just say 夜寝る前に歯を磨きます?

Yes! Japanese often omits the topic (or subject) when it’s clear from context. In everyday speech, you’d frequently just say 夜寝る前に歯を磨きます and native speakers would understand “I brush my teeth before going to sleep at night” without needing 私は. You include 私は only when you need to contrast or clarify who you’re talking about.

Why is the word order 夜寝る前に? Could we say 寝る前の夜に or 前に夜寝る?

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.

Why is 磨きます used instead of a verb like 掃く or another “brush” verb?

In Japanese, 磨く (migaku) is the standard verb for “to polish” or “to brush” when it comes to teeth and hair. 掃く (haku) means “to sweep” or “to brush” (like sweeping the floor). So to talk specifically about brushing your teeth, you always use 歯を磨く. Other objects have their own collocations—e.g. 靴を磨く (kutsu o migaku) “polish shoes.”

Can I replace 夜 with other time words, like 朝 or 寝る前 alone?

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.