Für den Salat nehme ich Avocado, Mais und ein paar Erbsen.

Because für always takes the accusative case.

Salat is masculine:

• nominative: der Salat
• accusative: den Salat

So für den Salat means for the salad.

German is a verb-second language in main clauses. That means the finite verb must come in the second position.

In this sentence, Für den Salat is placed first for emphasis, so the verb comes right after it:

• Für den Salat | nehme | ich | ...

If you start with ich, that is also correct:

• Ich nehme für den Salat Avocado, Mais und ein paar Erbsen.

Both are natural, but the original sentence puts a little more focus on for the salad.

Literally, yes, nehmen means to take. But in cooking or ingredient contexts, it often means something like:

• I use
• I go with
• I choose
• I’ll put in

So here nehme ich is best understood as I use or I’m using.

In German, food words are often used without an article when they are treated as ingredients or substances.

So:

• Ich nehme Avocado = I use avocado
• Ich nehme Mais = I use corn / sweetcorn

This sounds like naming ingredients, not necessarily counting individual items.

A sentence with an article can also be possible, especially if you mean a specific countable amount:

• Ich nehme eine Avocado = I use one avocado

So the version without an article sounds a bit more like an ingredient list.

Because these nouns are being viewed differently:

• Mais is usually treated as a mass noun: you do not normally count it piece by piece.
• Avocado here is being used like an ingredient name, similar to a substance.
• Erbsen are naturally thought of as many separate little peas, so the plural is very natural.

So the sentence mixes:

• ingredient/substance-style nouns: Avocado, Mais
• a counted small quantity: ein paar Erbsen

In modern German, ein paar usually means a few or some, not literally two.

So:

• ein paar Erbsen = a few peas

Although Paar originally relates to a pair, in this expression ein paar is a very common quantity phrase meaning just a small number of.

They are the direct objects of nehmen, so they are in the accusative case.

However, you do not really see a visible change here:

• Avocado stays Avocado
• Mais stays Mais
• ein paar Erbsen looks the same in nominative and accusative

So they are accusative by function, even though their forms do not visibly change much.

Yes. That is completely correct and natural.

Compare:

• Für den Salat nehme ich ... → stronger focus on for the salad
• Ich nehme für den Salat ... → more neutral, starting with I

German often changes word order to shift emphasis.

Yes. nehme is the present tense form of nehmen.

In German, the present tense can cover several English ideas depending on context:

• I use
• I’m using
• I will use

So in a cooking context, nehme ich can easily mean I use or I’ll use.

They are:

• der Salat — masculine
• die Avocado — feminine
• der Mais — masculine
• die Erbse — feminine
  plural: die Erbsen

In the sentence, only Salat shows its gender clearly through the article den.

German uses commas to separate items in a simple list, just like English:

• Avocado, Mais und ein paar Erbsen

In standard German, you normally do not put a comma before und in a simple series.

So this punctuation is exactly what you would expect in German.

Yes, depending on what you want to say.

• Avocado suggests the ingredient in a general sense: avocado
• eine Avocado suggests one whole avocado

So:

• Für den Salat nehme ich Avocado ... = ingredient-style wording
• Für den Salat nehme ich eine Avocado ... = more explicitly counted

Both can be correct; they just present the ingredient a little differently.