Ik koop kip en vis voor het avondeten.

Because the subject is ik (I). In the present tense, Dutch usually uses:

• ik koop = I buy
• jij/je koopt = you buy
• hij/zij koopt = he/she buys

So koop is the correct ik-form of the verb kopen.

In Dutch, ik is normally written with a lowercase i, unlike English I, which is always capitalized. It is only capitalized at the beginning of a sentence.

So:

• Ik koop... = correct at the start of the sentence
• ... en ik koop ... = lowercase in the middle of a sentence

Here, kip and vis are being used as general food words, like chicken and fish in English.

So:

• Ik koop kip en vis means I’m buying chicken and fish
• not necessarily a chicken and a fish

Dutch often leaves out the article when talking about food or substances in a general, uncountable way.

Compare:

• Ik koop kip. = I’m buying chicken.
• Ik koop een kip. = I’m buying a chicken. (the animal / one whole chicken)

The same idea works for vis.

It can mean both, depending on context.

• een kip usually means a chicken as an animal
• kip without an article often means chicken meat / chicken as food

In this sentence, because it appears together with vis and in the context of avondeten, it clearly means chicken as food.

Here, voor means for.

So:

• voor het avondeten = for dinner / for the evening meal

Dutch voor can also mean before, but the context tells you which meaning is intended.

Compare:

• Ik koop kip voor het avondeten. = I buy chicken for dinner.
• Voor het avondeten drink ik thee. = Before dinner I drink tea.

Same word, different meaning depending on structure and context.

Because avondeten is a het-word in Dutch. Dutch nouns take either de or het, and this is something learners often just have to memorize noun by noun.

So:

• het avondeten = the dinner / the evening meal

A useful thing to notice is that many neuter nouns take het, and avondeten is one of them.

That is just an idiomatic difference between the languages. Dutch often uses the article here:

• voor het avondeten

while English commonly says:

• for dinner

A very literal translation, for the dinner, would sound unnatural in normal English, but it is normal in Dutch.

Yes, avondeten is one word. Dutch forms compound nouns very often, much more freely than English.

Here:

• avond = evening
• eten = eating / food

Together:

• avondeten = evening meal / dinner

This is very typical Dutch spelling. English might prefer two words or a different word altogether, but Dutch commonly combines them.

This is normal main-clause Dutch word order:

• Ik = subject
• koop = finite verb
• kip en vis = object
• voor het avondeten = prepositional phrase

So the pattern is basically: subject + verb + object + extra information

That is very similar to English here:

• I buy chicken and fish for dinner.

Yes. Grammatically, that is also correct. It just changes the order of the items in the list.

• Ik koop kip en vis = I buy chicken and fish
• Ik koop vis en kip = I buy fish and chicken

Usually the difference is only emphasis, rhythm, or what the speaker thinks of first.

They are similar, but not always identical in tone or usage.

• avondeten is a very common everyday word for the evening meal
• diner can also mean dinner, but it can sound a bit more formal, specific, or meal-focused depending on context

In many ordinary situations, avondeten is the most natural choice for dinner.

Koop is pronounced roughly like kope in English, but with a pure long vowel and a crisp final p.

A rough guide:

• oo in koop sounds like a long o
• p is clearly pronounced at the end

So it is approximately:

• koop ≈ kope

But the Dutch vowel is usually tenser and cleaner than in many English accents.

Yes. The present tense in Dutch can be a little broader than a strict right now meaning.

This sentence can mean:

• I buy chicken and fish for dinner
• I’m buying chicken and fish for dinner

The exact time reference depends on context. If you want to make it more clearly about the future, Dutch often adds a time word or uses another construction, but this present-tense sentence is perfectly natural.