Ich frage mich, warum diese Endung in der Mehrzahl verschwindet.

In this sentence, sich fragen is an idiomatic reflexive expression meaning to wonder.

• Ich frage by itself usually means I ask.
• Ich frage mich means I ask myself, which is how German commonly expresses I wonder.

So:

• Ich frage mich, warum ... = I wonder why ...
• Ich frage, warum ... would sound more like I ask why ...

Mich is the reflexive pronoun for ich in the accusative case.

Here is the pattern:

• ich frage mich
• du fragst dich
• er/sie fragt sich
• wir fragen uns
• ihr fragt euch
• sie/Sie fragen sich

The verb expression is sich fragen. Even though English says I wonder, German uses this reflexive structure.

Because the expression sich fragen takes an accusative object.

You can think of it as:

• jemanden fragen = to ask someone
• sich fragen = to ask oneself / to wonder

So the reflexive pronoun follows the same pattern and stays in the accusative:

• Ich frage mich
• Du fragst dich

It is not dative here because this verb does not use a dative reflexive pronoun.

Because warum diese Endung in der Mehrzahl verschwindet is a subordinate clause.

In German, when a clause is introduced by a subordinating question word like warum, the conjugated verb normally goes to the end of that clause.

So:

• Main clause: Ich frage mich
• Subordinate clause: warum diese Endung in der Mehrzahl verschwindet

This is a very common German pattern:

• Ich weiß, dass er kommt.
• Sie erklärt, warum das wichtig ist.
• Wir sehen, wie er arbeitet.

German does not form embedded questions the same way English does.

In English, an embedded question often still shows do-support or other question structure:

• I wonder why this ending disappears.

German does not use do-support, and in subordinate clauses the verb goes to the end:

• ..., warum diese Endung in der Mehrzahl verschwindet.

So German keeps normal subordinate-clause word order rather than English-style question inversion.

Because Endung is a feminine noun, and here it is the subject of the subordinate clause, so it is in the nominative singular.

• die Endung = feminine singular nominative
• diese Endung = this ending

The verb verschwindet agrees with diese Endung, so diese Endung is the subject.

A quick comparison:

• diese Endung = nominative feminine singular
• dieser Endung = dative/genitive feminine singular, depending on context
• diesen would not fit with a feminine singular nominative noun

Endung means ending, usually in a grammar or word-formation sense.

It often refers to:

• a noun ending
• an adjective ending
• a verb ending
• any word ending or suffix-like ending in a grammatical explanation

So in this sentence, diese Endung means this ending in the grammatical form being discussed.

Because the sentence is talking about one specific ending that disappears when something is in the plural.

So the idea is:

• there is a particular ending in some form
• when the word changes to plural, that ending is no longer there

That is why German uses:

• diese Endung = singular, one ending
• in der Mehrzahl = in the plural

There is no contradiction: one ending disappears in plural forms.

In der Mehrzahl means in the plural.

Here, Mehrzahl is a feminine noun:

• die Mehrzahl

After in, German can use either accusative or dative, depending on meaning:

• accusative for direction/movement into something
• dative for location or state

Here it means in the plural as a grammatical state or category, not movement, so German uses the dative:

• in der Mehrzahl

Yes. Both are common.

• die Mehrzahl = a native German term for plural
• der Plural = the international grammatical term

So these are both natural:

• ... in der Mehrzahl verschwindet
• ... im Plural verschwindet

The second one is slightly more technical or grammar-textbook-like, but both are normal.

All three can mean why, and in many contexts they are interchangeable.

• warum = the most common neutral choice
• wieso = also very common in spoken language
• weshalb = a bit more formal or written in some contexts

So you could also say:

• Ich frage mich, wieso diese Endung in der Mehrzahl verschwindet.
• Ich frage mich, weshalb diese Endung in der Mehrzahl verschwindet.

But warum is the most straightforward and standard choice.

Yes, but the nuance is a little different.

• Ich frage mich, warum ... = I wonder why ...
• Ich wundere mich, warum ... = I am surprised / puzzled why ...

So sich fragen is often more neutral and simply expresses curiosity. Sich wundern suggests stronger surprise.

That means:

• Ich frage mich, warum diese Endung in der Mehrzahl verschwindet.
  = I am wondering about the reason.
• Ich wundere mich, warum diese Endung in der Mehrzahl verschwindet.
  = I find it surprising that this ending disappears in the plural.

It is neutral standard German.

Nothing in it is especially formal or especially casual:

• Ich frage mich is common in both speech and writing.
• warum is neutral.
• Mehrzahl is standard grammatical vocabulary.

So this sentence would sound natural in a textbook, a classroom, or ordinary conversation about grammar.