Der Gemüserest kommt auf den Kompost, nicht in den Restmüll.

Because the two nouns are in different cases.

• der Gemüserest is the subject of the sentence, so it is in the nominative case.
• den Restmüll comes after the preposition in here, and because there is a sense of movement/direction, German uses the accusative case.

So:

• der Gemüserest = nominative masculine
• den Restmüll = accusative masculine

This is a very common pattern in German: the article changes depending on the noun’s role in the sentence.

Yes. Although kommen usually means to come, German often uses it in contexts where English would naturally say go or belong.

In this sentence, kommt auf den Kompost means something like:

• goes on the compost
• belongs on the compost
• should be put on the compost

So this is a very natural German way to talk about where waste should be sorted.

This is about how German imagines the location.

• auf den Kompost = onto the compost
• in den Restmüll = into the residual waste / general trash

German uses:

• auf when something goes onto a surface or is thought of as being placed on top of something
• in when something goes inside a container or space

A compost heap is often thought of as a pile or surface, so auf den Kompost sounds natural. A trash bin is thought of as a container, so in den Restmüll is natural.

Because both auf and in are two-way prepositions. That means they can take either:

• dative for location
• accusative for direction/movement

Here the sentence describes where something is supposed to be put, so it uses direction, which means accusative.

That gives:

• auf den Kompost
• in den Restmüll

Compare:

• Der Rest liegt auf dem Kompost. = location, so dative
• Der Rest kommt auf den Kompost. = movement/destination, so accusative

Because nicht is negating that whole part of the sentence.

The sentence contrasts two destinations:

• auf den Kompost
• nicht in den Restmüll

So nicht is placed directly before the part being denied: in den Restmüll.

This is very common in German. The position of nicht often shows what exactly is being negated.

Here the meaning is:

• It goes on the compost
• not into the residual waste

Gemüserest is a compound noun:

• Gemüse = vegetables
• Rest = remainder, leftover, scrap

So der Gemüserest means something like:

• vegetable leftover
• vegetable scrap
• vegetable waste

German often combines nouns like this into one word. English usually uses separate words more often.

Yes, English often uses a plural here, but German can use the singular to talk about a material or category in a general way.

So der Gemüserest can mean:

• a vegetable leftover
• vegetable waste in general

German often uses the singular in this kind of instruction. It sounds like talking about the category rather than counting individual pieces.

Restmüll is a common German waste-sorting term. It means the waste that is left over after recyclable or compostable items have been separated.

It is often translated as:

• residual waste
• general waste
• non-recyclable trash

It is another compound noun:

• Rest = remainder
• Müll = garbage / trash

Because that is the subject and the thing the sentence is about.

German main clauses usually put the finite verb in second position, so if the sentence starts with the subject, the verb comes next:

• Der Gemüserest
  • kommt
    • ...

This is the most neutral, straightforward word order.

Yes, absolutely. That is very natural German too.

• kommt auf den Kompost = goes on the compost / gets put on the compost
• gehört auf den Kompost = belongs on the compost

Both work well in instructions about sorting waste.
gehört auf den Kompost may sound slightly more like a rule or classification, while kommt auf den Kompost sounds a bit more practical and action-oriented.

Yes, Kompost is masculine: der Kompost.

You see den Kompost here because auf is using the accusative case for direction.

So:

• nominative: der Kompost
• accusative: den Kompost

That article change is normal for masculine nouns.

Then German would usually use the dative after these prepositions.

For example:

• Der Gemüserest liegt auf dem Kompost.
• Der Gemüserest ist nicht im Restmüll.

Here there is no movement toward a destination. It is just describing where something is, so German uses:

• auf dem Kompost
• im Restmüll (in dem = im)

That is a useful contrast with the original sentence:

• kommt auf den Kompost = direction
• liegt auf dem Kompost = location