Die Mode ändert sich schnell.

Because Mode is a feminine noun in German. Its basic article is die.

• die Mode = fashion
• In the nominative singular, feminine nouns take die

So in this sentence, die Mode is the subject: Fashion changes quickly.

The infinitive is ändern = to change / to alter.

When it is conjugated for er/sie/es or for a singular noun like die Mode, it becomes:

• ich ändere
• du änderst
• er/sie/es ändert

So ändert is just the normal 3rd person singular form of ändern. The umlaut is already part of the verb itself: ändern, not something added only in this sentence.

Here, sich ändern means to change in the sense of to undergo change.

That is different from using ändern transitively, where something changes something else:

• Ich ändere den Plan. = I change the plan.
• Der Plan ändert sich. = The plan changes.

So:

• ändern = to change something
• sich ändern = to change, to become different

In Die Mode ändert sich schnell, fashion is not changing something else; fashion itself is changing. That is why sich is used.

Yes. Sich is the reflexive pronoun used with er/sie/es and plural forms like sie and Sie.

In this sentence, die Mode is singular and feminine, so it behaves like sie. That is why the reflexive pronoun is sich.

A quick comparison:

• Ich ändere mich = I change
• Du änderst dich = you change
• Die Mode ändert sich = fashion changes

So sich matches the subject in a reflexive construction.

In a normal German main clause, the conjugated verb usually comes in second position.

So the structure is:

• Die Mode = position 1
• ändert = position 2
• sich schnell = the rest of the sentence

That gives:

• Die Mode ändert sich schnell.

German does not usually place the reflexive pronoun before the conjugated verb in a standard main clause.

Compare:

• Das Wetter verbessert sich.
• Die Situation verändert sich langsam.

The finite verb stays in second position.

Because schnell is being used as an adverb here, not as an adjective before a noun.

• As an adjective before a noun, it would take endings:
  • die schnelle Mode
• As an adverb modifying a verb, it stays unchanged:
  • Die Mode ändert sich schnell.

Here, schnell describes how fashion changes, so it works like quickly in English.

Yes, you can. Sich verändern is also very common and often means to change or to undergo change.

Both are possible:

• Die Mode ändert sich schnell.
• Die Mode verändert sich schnell.

Often, verändern can sound a little more like change in form/condition over time, while ändern is a bit more general. But in many everyday contexts, the difference is small and both can work.

Yes. Mode is grammatically singular here.

Just like English fashion, German Mode can refer to the general concept as a mass or abstract noun:

• Die Mode ist wichtig.
• Die Mode ändert sich schnell.

If you wanted to talk about specific fashions or trends, you might use other wording, but in this sentence Mode is a singular abstract noun.

A helpful rough guide is:

• ändert ≈ EN-dert, but with the German ä, which is somewhat like the vowel in English bed, though not exactly the same.

A few points:

• ä sounds similar to e in many accents
• the r in standard German is usually not a strong English-style r
• the final -t is clearly pronounced

The whole sentence is roughly:

• Dee MOH-duh EN-dert zikh shnel

That is only an approximation, but it can help you get started.

No, that would sound wrong in standard German.

If you want to say that fashion changes quickly, you need the reflexive form:

• Die Mode ändert sich schnell.

Without sich, ändern normally expects a direct object:

• Die Firma ändert die Preise schnell. = The company changes the prices quickly.

So in your sentence, sich is necessary because the subject itself is what changes.

Because the sentence is not saying that fashion changes something else. It is saying that fashion itself changes.

That is why the structure is:

• subject: die Mode
• verb: ändert
• reflexive pronoun: sich
• adverb: schnell

So this is a complete sentence without a separate object. The reflexive construction already expresses the idea that the subject is undergoing the change.