Vi lægger gammelt græs i komposten, men kød kaster vi ud, så der ikke kommer skadedyr.

Because græs is a neuter noun in Danish: et græs.

In the indefinite singular, adjectives agree with the noun:

• en-words: gammel
• et-words: gammelt
• plural / definite: gamle

So:

• gammelt græs = old grass
• gammel ost = old cheese
• gamle blade = old leaves

Here græs is also being used as a mass noun, so singular is natural.

In this sentence, græs means grass as a material, or more specifically grass clippings / old grass, not one individual blade of grass.

Danish often uses the singular for mass or material nouns, just like English:

• græs = grass
• kød = meat
• vand = water

So gammelt græs is perfectly natural for general compost material.

Komposten is the definite form: the compost.

That usually means a specific compost heap, compost bin, or compost container that both speaker and listener know about — for example, the household compost.

• i komposten = in the compost / in the compost bin
• i en kompost = in a compost heap/bin, one of many

So i komposten sounds like the compost we use at home.

This is because Danish uses V2 word order in main clauses: the finite verb must come in the second position.

The normal order would be:

• Vi kaster kød ud.

But here kød is moved to the front for contrast:

• Vi lægger gammelt græs i komposten, men kød kaster vi ud ...

This gives something like:

• We put old grass in the compost, but meat, we throw out ...

So the fronting of kød emphasizes the contrast between grass and meat.

Because kaste ud is a verb + particle combination, similar to English throw out.

In Danish, particles like ud, op, ned, af are often separated from the verb in main clauses:

• Vi kaster kød ud.
• Han smider det væk.
• Jeg tager skoene af.

When kød is fronted, the particle still stays later in the clause:

• Kød kaster vi ud.

So this is normal Danish word order.

Because kød is being used in a general, uncountable sense: meat in general.

Compare:

• Kød er dyrt. = Meat is expensive.
• Vi spiser ikke kød. = We do not eat meat.
• Kød kaster vi ud. = Meat, we throw out.

If you meant a specific piece of meat, you might use a definite form or another more specific noun phrase, for example:

• kødet = the meat
• det kød, der lugter = the meat that smells

Here så means so or so that.

The phrase så der ikke kommer skadedyr expresses the result or purpose:

• ... so that pests do not come
• ... so there will not be pests

So the full logic is:

• We compost old grass,
• but we throw meat out,
• so that pests do not appear.

Here der is a dummy subject, similar to English there in sentences like:

• There are pests
• There come insects

In Danish, der kommer skadedyr means something like:

• pests appear
• there come pests
• there are pests showing up

This structure is very common when introducing the existence or appearance of something.

So:

• der kommer skadedyr = pests come / pests appear
• der ikke kommer skadedyr = pests do not come / pests do not appear

It is more natural here than a very literal structure with skadedyr as the subject at the front.

Because this is a subordinate clause introduced by så.

In Danish subordinate clauses, the usual order is:

conjunction + subject + sentence adverb + finite verb

So:

• så der ikke kommer skadedyr

Breakdown:

• så = conjunction
• der = subject
• ikke = negation
• kommer = finite verb
• skadedyr = logical subject/content being introduced

Compare a main clause:

• Der kommer ikke skadedyr.

In that main clause, the verb is in second position, so the order is different.

Because Danish often uses the present tense for:

• general truths
• habits
• routines
• instructions
• rules

So this sentence is not necessarily about one single moment right now. It sounds like a general household rule:

• We put old grass in the compost, but we throw meat out ...

English does the same thing:

• We recycle paper.
• We don’t compost meat.

Skadedyr means pest or vermin, depending on context.

It is a general word for animals or creatures that cause problems, especially around food, gardens, or waste. In this sentence it could mean things like:

• rats
• mice
• flies
• other unwanted animals/insects

A useful thing to know is that skadedyr has the same form in singular and plural:

• et skadedyr = a pest
• flere skadedyr = several pests

So in der ikke kommer skadedyr, the meaning is plural/general: so that pests do not come.

Because men is a coordinating conjunction, and in standard Danish writing it is normal to put a comma before it when it joins two clauses:

• Vi lægger gammelt græs i komposten, men kød kaster vi ud ...

There is also a comma before the final subordinate part:

• ..., så der ikke kommer skadedyr.

So the commas help show the structure clearly:

1. first main clause: Vi lægger gammelt græs i komposten
2. second main clause: men kød kaster vi ud
3. subordinate clause: så der ikke kommer skadedyr

Yes, lægger is very natural here.

lægge often means lay / put something somewhere, especially when talking about placing things into a container, pile, or area.

So:

• lægge i komposten = put in the compost

Other verbs are possible in some contexts:

• putte i komposten = put into the compost
  
  • a bit more informal
• smide i komposten = throw into the compost
  
  • more like tossing it in

But lægger sounds neutral and natural. It also contrasts nicely with kaster vi ud, which is stronger and clearly means throw away / discard.