Kannst du den Toast bitte umdrehen, bevor er zu dunkel wird?

Because Toast is the direct object of the verb umdrehen here, so it takes the accusative case.

• der Toast = nominative
• den Toast = accusative

You can think of it like this:

• Der Toast wird zu dunkel. = The toast is getting too dark.
• Kannst du den Toast umdrehen? = Can you turn the toast over?

So the article changes from der to den because the toast is being acted on.

Because the sentence uses the modal verb kannst.

In German, with a modal verb such as können, müssen, wollen, etc., the second verb stays in the infinitive and usually goes to the end of the clause.

So:

• Kannst du den Toast bitte umdrehen?

Structure:

• Kannst = finite/modal verb in second position
• du = subject
• den Toast = object
• bitte = polite particle
• umdrehen = infinitive at the end

Without a modal, the separable verb would split:

• Dreh den Toast bitte um.

Yes. umdrehen is a separable verb.

Its two parts are:

• um- = around / over
• drehen = to turn

When it is conjugated as the main verb in a normal main clause, the prefix separates:

• Ich drehe den Toast um.
• Dreh den Toast um!

But when it stays as an infinitive after a modal verb, it does not split:

• Ich kann den Toast umdrehen.
• Kannst du den Toast umdrehen?

That is why you see umdrehen as one word here.

In German, der Toast usually means toast or a slice of toast bread. Depending on context, it can refer to:

• toasted bread
• toast bread in general
• a slice of bread being toasted

In this sentence, it clearly means the piece of toast that needs to be turned over so it does not get too dark.

Also, the noun is masculine:

• der Toast
• den Toast
• dem Toast

Because bevor introduces a subordinate clause, and in German subordinate clauses, the conjugated verb goes to the end.

So:

• bevor = before
• er = he/it
• zu dunkel = too dark
• wird = becomes / gets

This is standard German word order after subordinating conjunctions such as:

• weil
• dass
• wenn
• bevor
• obwohl

Compare:

• Main clause: Er wird zu dunkel.
• Subordinate clause: ..., bevor er zu dunkel wird.

Er refers to der Toast.

Since Toast is a masculine noun in German, the pronoun that refers back to it is er.

So even though English would usually say it, German says er because grammatical gender matters:

• der Toast → er
• die Butter → sie
• das Brot → es

So bevor er zu dunkel wird literally means before he gets too dark, but in natural English we say before it gets too dark.

Because werden expresses a change of state: to become / to get.

• zu dunkel wird = gets too dark / becomes too dark
• zu dunkel ist = is too dark

In this sentence, the toast is in the process of getting darker, so wird is the natural choice.

Compare:

• Der Toast ist zu dunkel. = The toast is too dark.
  (state)
• Der Toast wird zu dunkel. = The toast is getting too dark.
  (change/process)

Bitte is quite flexible in German, and several positions can sound natural.

Your sentence:

• Kannst du den Toast bitte umdrehen?

This is very natural.

You could also say:

• Kannst du bitte den Toast umdrehen?

That is also natural.

The difference is usually very small. In everyday speech, bitte often goes somewhere in the middle of the sentence, not necessarily in one fixed position.

So as a learner, the important thing is:

• use bitte to sound polite
• place it in a natural middle position

Yes. In everyday German, Kannst du ... ? often functions as a polite request, just like Can you ... ? in English.

So although it literally asks about ability, it usually means:

• Could you ... ?
• Can you please ... ?

Other common request forms are:

• Kannst du den Toast bitte umdrehen?
• Könntest du den Toast bitte umdrehen?
  slightly softer / more polite
• Dreh den Toast bitte um.
  direct imperative

So this sentence is a normal, polite request.

Yes. If you want to be formal or polite with someone you do not know well, you would use Sie instead of du.

That gives:

• Können Sie den Toast bitte umdrehen, bevor er zu dunkel wird?

Changes:

• du → Sie
• kannst → können

Everything else stays basically the same.

Use:

• du for friends, family, children, many colleagues depending on context
• Sie for formal situations, strangers, customer-service situations, etc.

Yes, you could.

• bevor er zu dunkel wird = before it gets too dark
• bevor er verbrennt = before it burns / gets burned

The version with zu dunkel wird is a bit softer and refers to the toast becoming overly dark.
The version with verbrennt sounds stronger, as if it is about to be burned.

So both are possible, but they are not exactly identical in tone.

The sentence has a main clause followed by a subordinate clause.

Main clause:

• Kannst du den Toast bitte umdrehen

Subordinate clause:

• bevor er zu dunkel wird

A simple breakdown:

1. Kannst = finite verb in first position because this is a yes/no question
2. du = subject
3. den Toast = object
4. bitte = polite particle
5. umdrehen = infinitive at the end because of the modal verb
6. bevor = starts subordinate clause
7. er = subject of subordinate clause
8. zu dunkel = predicate adjective
9. wird = verb at the end of the subordinate clause

So this sentence shows two very common German patterns at once:

• modal verb + infinitive at the end
• subordinate clause + verb at the end