Learn Hardcore German: Bevor ich ins Bett gehe, putze ich mir die Zähne mit Zahnpasta. - Before I go to bed, I brush my teeth with toothpaste.

Main clause: Ich gehe ins Bett. (verb in 2nd position)
Subordinate clause with bevor: Bevor ich ins Bett gehe, … (same elements, but gehe moves to the end)

So the pattern is:

Bevor
- subject + objects/adverbials + verb (at the end)

Why does the second part say putze ich mir die Zähne and not ich putze mir die Zähne?

German main clauses follow the verb‑second (V2) rule: the conjugated verb must be in second position.

Here, the entire first clause Bevor ich ins Bett gehe counts as position 1 of the main sentence. That means in the main clause that follows:

Position 1: Bevor ich ins Bett gehe
Position 2: putze (the conjugated verb)
Then comes the subject: ich
Then other elements: mir die Zähne mit Zahnpasta

So we get:

Bevor ich ins Bett gehe, putze ich mir die Zähne mit Zahnpasta.

If you start with Ich instead, you would say:

Ich putze mir die Zähne mit Zahnpasta, bevor ich ins Bett gehe.

Both are correct; the difference is simply which part you put first.

Why is there a comma between Bevor ich ins Bett gehe and putze ich mir die Zähne …?

In German, a comma is required between a subordinate clause and the main clause.

Bevor ich ins Bett gehe, = subordinate clause (introduced by bevor)
putze ich mir die Zähne mit Zahnpasta. = main clause

German punctuation rules are stricter than English here: you must write this comma.

Why do we say ich putze mir die Zähne? What is mir doing here?

Mir is a reflexive dative pronoun. With many verbs about body care, German uses:

dative pronoun for the person
accusative noun for the body part

Examples:

Ich wasche mir die Hände. – I wash my hands.
Ich kämme mir die Haare. – I comb my hair.
Ich putze mir die Zähne. – I brush my teeth.

So:

die Zähne = direct object (accusative)
mir = indirect object (dative), referring to “to/for myself”

Literally: I clean myself the teeth.
Natural English: I brush my teeth.

What is the difference between mir and mich here?

Mir is dative; mich is accusative.

With putzen plus a body part, German uses:

dative reflexive for the person: mir
accusative for the body part: die Zähne

You cannot have two accusatives of this type here, so mich would be wrong.

Compare:

Ich putze mich. – I wash/clean myself (my whole body; no specific body part mentioned).
Ich putze mir die Zähne. – I brush my teeth (specific body part; so use mir
- body part).

Why is it die Zähne and not meine Zähne?

With body parts, German often uses the definite article (der/die/das) instead of a possessive (mein/dein/sein …) when the owner is clear from context, especially with a dative pronoun:

Ich wasche mir die Hände. (not usually meine Hände)
Er kämmt sich die Haare.
Ich putze mir die Zähne.

The mir already shows that the teeth belong to me, so meine Zähne is usually unnecessary.
You can say meine Zähne, but it sounds more emphatic or contrastive (e.g. “nicht deine, sondern meine Zähne”).

What does ins mean, and why not im Bett?

Ins is a contraction of in das:

in
- das Bett (accusative) → ins Bett

German uses in + accusative for movement into something:

Ich gehe ins Bett. – I go into bed.

German uses in + dative for location in something:

Ich bin im Bett. – I am in bed.
(in
- dem Bett → im Bett)

So:

ins Bett = into bed (motion, accusative)
im Bett = in bed (location, dative)

Can I say in das Bett gehe instead of ins Bett gehe?

Yes, grammatically it is correct to say:

Bevor ich in das Bett gehe, …

However, in everyday speech and writing, native speakers almost always use the contraction:

ins Bett

So ins Bett sounds much more natural and fluent.

Why are Bett, Zähne, and Zahnpasta capitalized?

In German, all nouns are capitalized, no matter where they appear in the sentence.

das Bett → Bett
die Zähne → Zähne
die Zahnpasta → Zahnpasta

Even if you don’t see the article, the noun itself is still capitalized.

Why is there no article in mit Zahnpasta? Could I say mit der Zahnpasta or mit einer Zahnpasta?

Zahnpasta is treated like a mass noun (like “water”, “sugar”) in this sentence. We are talking about toothpaste in general, not a specific tube.

mit Zahnpasta = with (some) toothpaste (general, typical way to say it)

You can use an article, but it changes the meaning:

mit der Zahnpasta – with the toothpaste (a specific one you both know about)
mit einer Zahnpasta – with a toothpaste (one kind of toothpaste among others; unusual in this context unless you’re comparing brands or types)

In a neutral, everyday sentence about brushing teeth, mit Zahnpasta is the normal form.

Can I change the order and say Ich putze mir die Zähne mit Zahnpasta, bevor ich ins Bett gehe?

Yes, that is completely correct and very common:

Ich putze mir die Zähne mit Zahnpasta, bevor ich ins Bett gehe.

Meaning and grammar stay the same. The difference is only:

What you put first in the sentence (main clause vs. time clause).
The intonation: starting with Bevor … emphasizes the time condition; starting with Ich putze … emphasizes the action first.

Why are both verbs (gehe, putze) in the present tense, even if this happens later (for example, tonight)?

German often uses the present tense to talk about future events when the time is clear from context or from time expressions (like bevor, morgen, später):

Morgen gehe ich ins Bett. – I’ll go to bed tomorrow.
Bevor ich ins Bett gehe, putze ich mir die Zähne. – Before I go to bed, I (will) brush my teeth.

You could form a future tense with werden:

Bevor ich ins Bett gehen werde, werde ich mir die Zähne putzen.

but this sounds heavy and is usually unnecessary in normal conversation. The simple present is preferred here.