In die Ablage links kommt alles, was heute noch beantwortet werden muss.

Because in is a two-way preposition in German: it can take either the accusative or the dative, depending on the meaning.

• accusative = movement toward/into something
• dative = location inside something

Here, the idea is that things are to be put into the tray, so German uses the accusative:

• in die Ablage = into the tray

If it meant that the things are already located there, you would use the dative:

• In der Ablage links ist alles. = Everything is in the tray on the left.

Here, die Ablage means something like:

• tray
• in-tray
• file tray
• sorting tray

In an office context, it often refers to a physical place where papers are put for sorting or processing.

So die Ablage links is probably the tray on the left.

German main clauses follow the verb-second rule: the conjugated verb must be in the second position.

The sentence begins with the prepositional phrase:

• In die Ablage links

That takes the first position, so the verb has to come next:

• In die Ablage links kommt alles ...

The subject alles comes after the verb.

A more neutral order would be:

• Alles kommt in die Ablage links.

Both are correct. The original version puts extra focus on where everything goes.

Because alles is grammatically singular in German.

Even though it refers to many things in meaning, it is treated as a singular neuter pronoun:

• alles kommt
• not alles kommen

This is similar to English everything goes, not everything go.

links means on the left or left-hand.

In this sentence, it describes which tray is meant:

• die Ablage links = the tray on the left

This kind of phrasing is common in everyday German, especially when pointing out location in a room, office, shelf system, etc.

You could also say:

• die linke Ablage = the left tray

The version with links sounds a bit more like spoken or practical labeling language.

After words like:

• alles = everything
• etwas = something
• nichts = nothing
• vieles = much / many things

German very often uses the relative pronoun was, not das.

So:

• alles, was ... = everything that ...

This is standard German.

Compare:

• alles, was ich weiß = everything that I know

Using das here would sound wrong in standard German.

heute noch means something like:

• still today
• before the end of today
• today yet

The word noch adds the idea that there is still time left today, but the action must happen before today is over.

So:

• was heute noch beantwortet werden muss
  = that still has to be answered today

Without noch, heute would just mean today, without that extra sense of urgency or remaining time.

This is a combination of:

• a passive construction
• plus a modal verb

Basic parts

• beantworten = to answer
• beantwortet werden = to be answered
• beantwortet werden muss = must be answered

Why this order?

Because this is a subordinate clause introduced by was, the finite verb goes to the end. With a passive + modal combination, German puts the verbal elements together at the end:

• ..., was heute noch beantwortet werden muss.

Literally:

• ..., what today still answered be must

Natural English:

• ..., that still has to be answered today

It could be muss beantwortet werden in a main clause, but not here.

Compare:

Main clause

• Es muss heute noch beantwortet werden.
  = It must still be answered today.

Subordinate clause

• ..., was heute noch beantwortet werden muss.
  = ..., that still has to be answered today.

In subordinate clauses, German sends the finite verb to the end, so muss comes last.

Yes, absolutely. That version is very natural.

It may even be easier for learners to understand because the subject comes first:

• Alles, was heute noch beantwortet werden muss, kommt in die Ablage links.

The original sentence:

• In die Ablage links kommt alles, was heute noch beantwortet werden muss.

is just a different word order with stronger emphasis on the destination.

It contains both an active main clause and a passive relative clause.

Main clause

• In die Ablage links kommt alles ...
• This is active in form.
• alles is the subject of kommt.

Relative clause

• ..., was heute noch beantwortet werden muss
• This is passive.
• The focus is not on who answers it, but on the fact that it must be answered.

So the sentence as a whole mixes active structure and passive structure.

In this context, kommt is often best translated naturally as goes in English, even though the basic verb is kommen = to come.

German often uses kommen in sorting or placement contexts to mean something like:

• belongs in
• goes into
• is to be put into

So:

• In die Ablage links kommt alles ...

means naturally:

• Everything ... goes in the tray on the left.

not necessarily that the papers physically come there by themselves.

Yes. beantworten is a transitive verb, so it takes a direct object in the accusative.

Examples:

• Ich beantworte die E-Mail. = I answer the email.
• Er beantwortet die Frage. = He answers the question.

In your sentence, the thing being answered is represented by was:

• alles, was ... beantwortet werden muss

That was is the thing that must be answered.