Wir bauen das Elektrogerät zusammen.

Because the full verb is zusammenbauen, which is a separable verb.

In a normal main clause, German splits that verb into:

• bauen = the conjugated part, which goes in the usual verb position
• zusammen = the separable prefix, which goes to the end

So:

• Wir bauen das Elektrogerät zusammen.

But when the verb is not split, it stays as one word:

• Wir wollen das Elektrogerät zusammenbauen.
• ..., weil wir das Elektrogerät zusammenbauen.

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

Sort of, but in this sentence it is mainly part of the verb zusammenbauen.

On its own, zusammen can mean together, but here bauen ... zusammen means assemble / put together.

So the sentence does not just mean We build the electrical device together in a loose sense. It more specifically means We assemble the electrical device.

Because the subject is wir, the idea of doing it jointly is already there anyway.

Bauen by itself usually means to build or construct.

But zusammenbauen is more specific: it means to assemble, put together, or build from parts.

So in this sentence:

• bauen alone would sound more like build
• zusammenbauen makes it clear that the device is being assembled

That is why English often translates this sentence with assemble rather than just build.

Because Elektrogerät is a neuter noun, so its article is das.

A useful rule for German compounds is:

• the last part of the compound decides the gender

Here the last part is Gerät, and das Gerät is neuter.
So:

• das Gerät
• das Elektrogerät

In this sentence, das Elektrogerät is also the direct object, so it is in the accusative case. For neuter singular nouns, the article is still das, so nominative and accusative look the same here.

Because German usually writes compound nouns as one word.

So English might write:

• electrical device

But German combines the parts:

• Elektrogerät

This is completely normal in German. Very long compound nouns are common.

Also, like all German nouns, Elektrogerät is capitalized.

It is present tense.

German present tense can cover several ideas that English often separates:

• We assemble the electrical device.
• We are assembling the electrical device.
• sometimes even We will assemble the electrical device, if the context makes that clear

So German uses the present tense more broadly than English does.

Because the subject is wir.

The present-tense forms of bauen are:

• ich baue
• du baust
• er/sie/es baut
• wir bauen
• ihr baut
• sie/Sie bauen

So with wir, the correct form is bauen.

The basic pattern here is very typical German main-clause word order:

• Wir = subject
• bauen = finite verb in second position
• das Elektrogerät = object
• zusammen = separable prefix at the end

So the standard sentence is:

• Wir bauen das Elektrogerät zusammen.

But German can move elements for emphasis, as long as the finite verb stays in the second position:

• Das Elektrogerät bauen wir zusammen.

That sounds more like The electrical device is what we’re assembling.

What you usually cannot do is place zusammen just anywhere if it is part of the separable verb. In a main clause, it normally belongs at the end.