Spinat kommt heute nicht auf den Tisch, weil mein Neffe lieber Mais isst.

It is partly literal, but in this sentence it works as a common idiom.

• Literal meaning: to come onto the table
• Natural meaning here: to be served / to be on the menu

So Spinat kommt heute nicht auf den Tisch means that spinach is not going to be served today.

This expression is very common with food, but it can also be used more broadly for topics that are being discussed:

• Das Thema kommt heute nicht auf den Tisch. = That topic is not being brought up today.

Because auf is a two-way preposition in German.

With two-way prepositions:

• accusative is used for direction / movement toward somewhere
• dative is used for location / position

Here, the idea is coming onto the table, so German uses the accusative:

• auf den Tisch = onto the table
• auf dem Tisch = on the table

Compare:

• Das Buch liegt auf dem Tisch. = The book is on the table.
• Ich lege das Buch auf den Tisch. = I put the book onto the table.

Even though this sentence is idiomatic, German still keeps that movement pattern.

Because food words like these are often used without an article when talking about them in a general sense or as a menu item.

So:

• Spinat kommt heute nicht auf den Tisch.
• Mein Neffe isst lieber Mais.

This is similar to English using bare nouns:

• Spinach isn’t being served today.
• He prefers corn.

You could use an article in other contexts, but the meaning changes:

• Der Spinat ist schon kalt. = The spinach is already cold.
  Now you mean a specific portion/dish of spinach.

The placement of nicht depends on what is being negated.

Here, nicht negates the whole idea of coming onto the table / being served:

• Spinat kommt heute nicht auf den Tisch.

A useful rule:

• nicht often comes before the part it most directly negates
• but it usually comes after time expressions like heute

So this sentence is structured very naturally:

• Spinat = topic/subject
• kommt = verb in second position
• heute = time expression
• nicht = negation
• auf den Tisch = the expression being negated

If you moved nicht, the emphasis could change or the sentence might sound unnatural.

Because weil introduces a subordinate clause, and in standard German the conjugated verb goes to the end of that clause.

Main clause:

• Spinat kommt heute nicht auf den Tisch

Subordinate clause with weil:

• weil mein Neffe lieber Mais isst

This is one of the most important German word-order patterns.

Compare:

• Mein Neffe isst lieber Mais.
  In a normal main clause, isst is in second position.
• ..., weil mein Neffe lieber Mais isst.
  After weil, the verb moves to the end.

Because lieber expresses a preference: rather / more gladly.

German uses forms of gern like this:

• gern = gladly / likes to
• lieber = rather / prefers
• am liebsten = most of all

So:

• Mein Neffe isst Mais gern. = My nephew likes eating corn.
• Mein Neffe isst lieber Mais. = My nephew prefers corn.

In this sentence, the meaning is comparative: he prefers corn instead of spinach, so lieber is the right choice.

Because mein Neffe is the subject of the clause, so it must be in the nominative case.

In:

• weil mein Neffe lieber Mais isst

the nephew is the one doing the eating, so he is the subject.

A detail that often confuses learners: Neffe is a weak noun. That means in many cases it adds -n or -en:

• nominative: der Neffe
• accusative: den Neffen
• dative: dem Neffen
• genitive: des Neffen

So mein Neffe is correct here because it is nominative singular.

Yes, but German often uses the present tense for the near future, especially when the time is already clear.

So:

• Spinat kommt heute nicht auf den Tisch literally uses the present tense, but naturally means:
• Spinach isn’t being served today or
• Spinach won’t be on the table today

The word heute already tells you when this happens, so German does not need a separate future form.

This is very common:

• Ich gehe morgen ins Kino. = I’m going to the cinema tomorrow.
• Wir essen heute Abend zu Hause. = We’re eating at home tonight.

Yes, but the structure changes.

With weil, you use a subordinate clause, so the verb goes to the end:

• ..., weil mein Neffe lieber Mais isst.

With denn, you use a main-clause word order:

• Spinat kommt heute nicht auf den Tisch, denn mein Neffe isst lieber Mais.

So both can mean because, but:

• weil = subordinate clause, verb at the end
• denn = coordinating conjunction, normal main-clause order

In everyday German, weil is often more common.

Yes. German word order is flexible, as long as the verb stays in the correct position for the clause type.

The original sentence is:

• Spinat kommt heute nicht auf den Tisch

You could also say:

• Heute kommt Spinat nicht auf den Tisch.

Both are correct, but the emphasis changes slightly:

• Spinat first: the sentence is mainly about spinach
• Heute first: the sentence emphasizes today

German often puts the most important or topical element in the first position, but the conjugated verb still has to stay in second position in a main clause.

Because the subject is Spinat, which is grammatically singular.

Even though spinach can feel like a substance rather than a single object, German still treats Spinat here as a singular noun:

• der Spinat
• Spinat kommt ...

So the verb must be singular:

• kommt

If the subject were plural, then you would use kommen:

• Die Gemüsegerichte kommen heute nicht auf den Tisch.

Yes. In the clause

• weil mein Neffe lieber Mais isst

the structure is:

• mein Neffe = subject
• isst = verb
• Mais = direct object in the accusative

With nouns like Mais, you often do not see a visible case ending, so nominative and accusative look the same. But functionally, Mais is the thing being eaten, so it is the direct object.

If you used a masculine noun with an article, the accusative would be easier to see:

• weil mein Neffe lieber den Apfel isst

Here den Apfel clearly shows the accusative.