Das Ersatzteil passt zum Elektrogerät.

Because zu is followed by the dative case, and das becomes dem in the dative.

So:

• das Elektrogerät = nominative/accusative
• dem Elektrogerät = dative

Then zu dem is very commonly contracted to zum:

• zu dem Elektrogerät → zum Elektrogerät

So passt zum Elektrogerät is the normal form.

Here, zum means something like with, for, or to in the sense of compatibility.

The verb passen zu means to match, to go with, or to be suitable for.

So:

• Das Ersatzteil passt zum Elektrogerät. = The spare part fits the electrical device. = The spare part is compatible with the electrical device.

It is not really about movement to something, even though zu often means to in other contexts.

Because the subject is das Ersatzteil, which is singular.

• das Ersatzteil = the spare part
• passt = singular form of passen

Compare:

• Das Ersatzteil passt. = The spare part fits.
• Die Ersatzteile passen. = The spare parts fit.

So the verb agrees with das Ersatzteil, not with Elektrogerät.

The dictionary form is passen.

In this sentence, passt is the 3rd person singular present tense form:

• ich passe
• du passt
• er/sie/es passt
• wir passen
• ihr passt
• sie/Sie passen

Since das Ersatzteil is grammatically es, German uses passt.

Yes, both are compound nouns, which is very common in German.

• Ersatzteil = Ersatz

  • Teil
  • Ersatz = replacement/spare
  • Teil = part
  • so: spare part

• Elektrogerät = Elektro

  • Gerät
  • Elektro = electrical/electric
  • Gerät = device/appliance
  • so: electrical device/appliance

A useful rule: in German compounds, the last part usually determines the gender.

• das Teil → das Ersatzteil
• das Gerät → das Elektrogerät

That is why both nouns here are neuter.

Because in German, all nouns are capitalized.

So in this sentence:

• Das is capitalized because it is the first word of the sentence
• Ersatzteil is capitalized because it is a noun
• Elektrogerät is capitalized because it is also a noun

This is standard German spelling.

Das Ersatzteil is in the nominative case because it is the subject of the sentence.

You can think of it this way:

• Who/what fits? → Das Ersatzteil

That makes it the subject, so nominative is used.

Then zum Elektrogerät is a prepositional phrase with zu, so Elektrogerät is in the dative.

Usually, no—not in standard German for this meaning.

With passen, the usual pattern for compatibility is:

• passen zu + dative

So:

• Das Ersatzteil passt zum Elektrogerät.

That is the natural way to say The spare part fits/goes with the device.

German does use für in many contexts, but passen für is not the normal standard choice here. If you want to say is suitable for, you could also say:

• Das Ersatzteil ist für das Elektrogerät geeignet.

That means The spare part is suitable for the electrical device.

No. Passen has several related meanings.

Common ones include:

• to fit physically
  
  • Der Schlüssel passt ins Schloss.
• to suit / to be suitable
  • Das passt gut.
• to match / to go with
  • Die Schuhe passen zum Kleid.

In your sentence, it most likely means to fit / be compatible with the device.

So the idea is not just size, but also appropriateness or compatibility.

Because this is a normal German main clause, and in a main clause the finite verb usually comes in second position.

So the structure is:

• Das Ersatzteil = first element
• passt = finite verb in second position
• zum Elektrogerät = the rest

This is standard German word order.

You could change the emphasis by moving another element to the front, but the verb still stays second:

• Zum Elektrogerät passt das Ersatzteil.

That sounds more marked, with emphasis on zum Elektrogerät.

The plural forms are:

• das Ersatzteil → die Ersatzteile
• das Elektrogerät → die Elektrogeräte

Examples:

• Die Ersatzteile passen zum Elektrogerät.
• Diese Elektrogeräte sind neu.

Notice that in the plural, the article becomes die for both nouns.

A simple approximate pronunciation is:

dahs er-ZAHTS-tyle pahst tsoom eh-LEK-troh-guh-rEHT

A few points:

• Ersatzteil has the ts sound in Ersatz
• passt ends with a clear st
• zum sounds like tsoom
• Gerät is stressed on the second syllable: ge-RÄT

If you want to sound more natural, try saying it smoothly as one phrase:

Das Ersatzteil passt zum Elektrogerät.