Im Salat sind Mais und Avocado, und darüber gebe ich nur wenig Öl.

Im is the contraction of in dem.

• in dem Salat → im Salat

German uses this contraction very often when it sounds natural.

It is dem because in takes the dative here: the corn and avocado are located in the salad, not moving into it.

• location: im Salat
• movement into something: in den Salat

So here im Salat means in the salad.

Yes. This is a very natural German structure.

German often does not need a direct equivalent of English there is / there are. Instead, it can simply put the location first and then use sein:

• Im Salat sind Mais und Avocado.

A more straightforward version would be:

• Mais und Avocado sind im Salat.

Both are correct. Starting with Im Salat puts the focus on what is in the salad.

Because the subject is Mais und Avocado, which is plural.

Even though Mais and Avocado are each singular by themselves, X und Y together count as more than one thing, so German uses the plural verb:

• Mais und Avocado sind ...

Compare:

• Die Avocado ist im Salat.
• Mais und Avocado sind im Salat.

Because in this kind of ingredient-style statement, German often uses bare nouns without articles.

That is especially natural when talking about food ingredients in a general way:

• Im Salat sind Mais und Avocado.

This sounds like listing what the salad contains.

A few reasons this feels natural:

• Mais is often treated like a mass/ingredient noun.
• Avocado can also appear without an article when used as an ingredient name rather than as a specific individual fruit.

If you wanted to be more specific, you could use an article or a quantity word:

• Im Salat ist eine Avocado.
• Im Salat sind der Mais und die Avocado aus dem Kühlschrank.
  (now you mean specific ones)

They are:

• der Mais
• die Avocado
• das Öl

So if you used them with articles, you would say:

• der Mais
• die Avocado
• das Öl

But in the sentence you were given, the articles are simply omitted because the words are being used as ingredients.

Here darüber means something like over that, on top of that, or over it.

It refers back to the salad as a thing. In German, when a preposition refers back to a thing rather than a person, German very often uses a pronominal adverb like:

• darüber
• darauf
• damit
• daran

So instead of über es, German usually prefers darüber.

That is why:

• darüber gebe ich nur wenig Öl

sounds natural, while

• über es gebe ich nur wenig Öl

sounds awkward.

Because German main clauses put the finite verb in second position.

In the second clause, darüber has been moved to the front for emphasis or flow:

• darüber = first position
• gebe = second position
• ich = then comes after the verb

So:

• ... und darüber gebe ich nur wenig Öl.

This is standard German word order.

You could also say:

• ... und ich gebe darüber nur wenig Öl.

That is also grammatical, but it gives a slightly different emphasis. With darüber first, the sentence links more smoothly to the previous clause.

Not in the sense of giving something to a person.

In cooking and everyday food language, geben often means:

• to add
• to put
• to pour

So:

• darüber gebe ich nur wenig Öl

means something like:

• I only add a little oil on top
• I put only a little oil over it

This use of geben is very common in recipes and food descriptions.

Nur wenig Öl means only a little oil or just a small amount of oil.

There is no article before Öl because Öl is an uncountable noun here, and it is already being quantified by wenig.

So the structure is:

• wenig Öl = little oil
• nur wenig Öl = only a little oil

The word Öl is the direct object, so it is in the accusative, but because it is a mass noun used in an indefinite amount, no article is needed.

Compare:

• Ich kaufe Öl.
• Ich gebe wenig Öl darüber.
• Ich nehme das Öl aus der Flasche.
  (specific oil)

Also, wenig Öl and ein wenig Öl are close in meaning, but:

• wenig Öl can sound a bit more restrictive: not much oil
• ein wenig Öl often sounds a bit softer: a little oil

Usually, no. In German, a comma before und is often not required.

So many writers would simply write:

• Im Salat sind Mais und Avocado und darüber gebe ich nur wenig Öl.

However, when und connects two full main clauses, some people use a comma to make the sentence easier to read:

• Im Salat sind Mais und Avocado, und darüber gebe ich nur wenig Öl.

So the comma here is mainly about clarity and style, not meaning.

Because in German, all nouns are capitalized.

So:

• der Salat
• der Mais
• die Avocado
• das Öl

This is one of the most noticeable spelling rules in German. Adjectives, verbs, and most other word types are not capitalized unless they begin the sentence.