Der Salat schmeckt besser, wenn Mais und Bohnen noch ein bisschen warm sind.

Because Der Salat is the subject of the sentence.

• der Salat = nominative case
• den Salat = accusative case

In this sentence, the salad is the thing that does the verb schmeckt:

• Der Salat schmeckt besser. = The salad tastes better.

So German uses the nominative form der.

schmecken means to taste or to taste like / be tasty.

So:

• Der Salat schmeckt besser = The salad tastes better

English learners sometimes expect something like ist besser (is better), but schmecken is more natural when talking about flavor.

Compare:

• Der Salat ist besser. = The salad is better.
  This could mean better in a general sense.
• Der Salat schmeckt besser. = The salad tastes better.
  This specifically refers to flavor.

Because besser is being used as an adverb/predicative form, not as an adjective directly before a noun.

Here it describes how the salad tastes:

• Der Salat schmeckt besser.

German adjectives only take endings when they come before a noun:

• ein besserer Salat = a better salad

But after verbs like sein, werden, schmecken, bleiben, adjectives usually stay in the basic form:

• Der Salat ist gut.
• Der Salat schmeckt besser.
• Die Bohnen sind warm.

Here wenn means when in the sense of whenever / under the condition that.

So the sentence means:

• The salad tastes better when the corn and beans are still a little warm.

German wenn can often mean:

• when for repeated or general situations
• if for conditions

In this sentence, both ideas are close:

• when the corn and beans are still warm
• if the corn and beans are still warm

English usually prefers when here.

Because the clause introduced by wenn is a subordinate clause, and in German subordinate clauses the conjugated verb goes to the end.

Main clause:

• Der Salat schmeckt besser

Subordinate clause:

• wenn Mais und Bohnen noch ein bisschen warm sind

This is a very important German word-order rule:

• Ich bleibe zu Hause, wenn es regnet.
• Wir essen, wenn er kommt.
• Der Salat schmeckt besser, wenn Mais und Bohnen warm sind.

Because Mais und Bohnen are two things joined by und, so together they form a plural subject.

• Mais = corn
• Bohnen = beans

So German uses the plural verb:

• Mais und Bohnen sind warm. = Corn and beans are warm.

If there were only one thing, you would use ist:

• Der Mais ist warm.
• Die Bohne ist warm.

Because warm is a predicate adjective here, not an adjective directly before a noun.

After verbs like sein, German adjectives do not take endings:

• Die Bohnen sind warm.
• Der Salat ist gut.

But before a noun, they do take endings:

• warme Bohnen = warm beans
• ein warmer Salat = a warm salad

So:

• Die Bohnen sind warm. ✅
• die warmen Bohnen ✅

Here noch means still.

• noch warm = still warm

So the idea is that the corn and beans are warm at that moment, perhaps because they were recently cooked and have not cooled down yet.

Common meanings of noch include:

• still
  
  • Er schläft noch. = He is still sleeping.
• yet / another / more, depending on context
  
  • noch ein Stück Kuchen = another piece of cake

In this sentence, still is the natural meaning.

ein bisschen means a little or a bit.

So:

• ein bisschen warm = a little warm / slightly warm

It softens the statement. The corn and beans do not need to be hot—just somewhat warm.

Similar expressions:

• ein wenig warm = a little warm
• leicht warm = slightly warm

German often leaves out articles when talking about foods or ingredients in a general sense.

So:

• Mais und Bohnen = corn and beans

This sounds natural when speaking about ingredients in a dish.

You could also see articles in other contexts, for example when referring to specific items:

• der Mais = the corn
• die Bohnen = the beans

But in recipe-like or food-description contexts, article omission is common:

• Tomaten und Zwiebeln schmecken gut zusammen.
• Mais und Bohnen sind noch warm.

Yes. German can put the subordinate clause first:

• Wenn Mais und Bohnen noch ein bisschen warm sind, schmeckt der Salat besser.

This means exactly the same thing.

But notice the word order in the main clause after that: German keeps the verb in second position, so you get:

• ..., schmeckt der Salat besser not
• ..., der Salat schmeckt besser

That is a very common German pattern:

• Wenn ich Zeit habe, komme ich.
• Wenn es regnet, bleiben wir zu Hause.

Yes, besser is the comparative form of gut.

• gut = good
• besser = better
• am besten = best

In this sentence, the comparison is implicit. It means something like:

• The salad tastes better than it would otherwise
• The salad tastes better than when the corn and beans are cold

German often leaves that unstated if it is obvious from context.

A rough English-friendly guide is:

• schmeckt ≈ shmekt

A few details:

• sch sounds like English sh
• e is short, like in bed
• ck is a hard k sound
• the final t is pronounced clearly

So the word is short and crisp: shmekt.

Because the sentence is not talking about eating a salad as an object. It is making a statement about the salad.

Compare:

• Ich esse den Salat. = I am eating the salad.
  Here den Salat is the object.
• Der Salat schmeckt besser. = The salad tastes better.
  Here der Salat is the subject.

So the case depends on the job the noun is doing in the sentence, not just on the word itself.