Der Brief muss heute noch abgeschickt werden.

Because der Brief is the grammatical subject of the sentence.

This sentence is in the passive voice, so the thing receiving the action becomes the subject:

• Active: Jemand muss den Brief heute noch abschicken.
• Passive: Der Brief muss heute noch abgeschickt werden.

In the passive version, der Brief is in the nominative case, so it appears as der.

Yes. It is a passive sentence.

German forms this kind of passive with:

• werden
  • past participle

Here:

• abgeschickt = past participle of abschicken
• werden = passive auxiliary

So abgeschickt werden means to be sent / to be mailed.

A natural active version would be:

• Jemand muss den Brief heute noch abschicken.

The passive is used because the sentence focuses on the letter, not on who sends it.

Because the sentence combines:

• a modal verb: müssen = must / have to
• the passive construction: abgeschickt werden = be sent

So the structure is:

• Der Brief = the letter
• muss = must
• heute noch = today, still / before the day ends
• abgeschickt werden = be sent

Together, muss ... abgeschickt werden means must be sent.

No. Here werden is not a future marker.

German werden can have different jobs:

• future: Ich werde gehen. = I will go.
• passive auxiliary: Der Brief wird geschickt. = The letter is being sent / is sent.

In Der Brief muss heute noch abgeschickt werden, werden is part of the passive, so it means be, not will.

Because German often puts non-finite verb forms toward the end of the clause.

In this sentence:

• muss is the finite verb, so it goes in the second position
• the remaining verb forms go to the end:
  • abgeschickt
  • werden

So the basic pattern is:

• Subject + finite verb + other elements + verb cluster

That gives:

• Der Brief | muss | heute noch | abgeschickt werden.

This is very normal in German when you have a modal verb plus a passive construction.

Because the verb is abschicken, not just schicken.

Abschicken is a separable verb:

• abschicken = to send off / mail / dispatch

Its past participle is formed as:

• ab
  • ge
    • schickt = abgeschickt

This is typical for separable verbs in German:

• anrufen → angerufen
• aufmachen → aufgemacht
• abschicken → abgeschickt

So abgeschickt is the correct participle of abschicken.

Here noch means something like:

• still
• yet
• before it is too late
• before the day is over

In heute noch, it usually means:

• still today
• today before the day ends

So Der Brief muss heute noch abgeschickt werden suggests urgency: the letter has to be sent later today, but not after today.

Because heute noch adds a stronger sense of deadline.

Compare:

• heute = today
• heute noch = still today / today before the day is over

So:

• Der Brief muss heute abgeschickt werden = The letter must be sent today.
• Der Brief muss heute noch abgeschickt werden = The letter must still be sent today; there is a sense that time is running out.

For an English speaker, heute noch often feels like still today or by the end of today.

Because passive sentences often leave out the doer of the action when it is:

• unknown
• obvious
• unimportant
• less important than the thing affected

Here the important information is what must happen to the letter, not who will do it.

If you wanted to mention the doer, you could add it, for example:

• Der Brief muss heute noch von Anna abgeschickt werden.

But in many everyday situations, German prefers the passive without naming the agent.

Yes. That is the active version, and it is completely correct.

Compare the focus:

• Jemand muss den Brief heute noch abschicken.
  Focus: the person doing the action
• Der Brief muss heute noch abgeschickt werden.
  Focus: the letter and what needs to happen to it

German uses the passive very naturally when the result or required action matters more than the person responsible.

No. They are related, but not the same.

• abgeschickt werden = to be sent
  This focuses on the action or process.
• abgeschickt sein = to have been sent / to be already sent
  This focuses on the completed state.

Compare:

• Der Brief muss heute noch abgeschickt werden.
  The letter must be sent today.
• Der Brief muss heute noch abgeschickt sein.
  The letter must already be sent by today / by the end of today.

So werden is about the action happening, while sein is about the finished result.