Der Ofen scheint schon heiß genug zu sein, also müssen wir ihn nicht länger vorheizen.

Because der Ofen is the subject of the first clause — it is the thing that seems hot enough.

• der Ofen = nominative singular masculine
• den Ofen = accusative singular masculine

In Der Ofen scheint ..., the oven is doing the grammatical action of scheinen (to seem), so it must be in the nominative case.

This is a very common structure with scheinen.

• scheinen = to seem
• zu sein = to be

So scheint heiß genug zu sein literally works like seems to be hot enough.

German often uses:

• scheinen + zu + infinitive

Examples:

• Er scheint müde zu sein. = He seems to be tired.
• Sie scheint recht zu haben. = She seems to be right.

In your sentence, zu sein goes to the end because infinitive phrases usually go to the end of the clause.

Here schon most naturally means already.

So scheint schon heiß genug zu sein means the oven seems already hot enough.

Depending on tone and context, schon can sometimes add a slight sense like by now or as it is already, but already is the clearest basic meaning here.

In German, genug normally comes after the adjective or adverb it modifies.

So:

• heiß genug = hot enough
• schnell genug = fast enough
• groß genug = big enough

This is different from English, where enough also comes after the adjective, so the pattern is actually similar:

• hot enough
• big enough

Here zu is the German infinitive marker, similar to English to in to be.

So:

• sein = be
• zu sein = to be

After scheinen, German commonly uses this zu + infinitive structure:

• scheint zu sein
• scheint zu funktionieren
• scheint zu stimmen

Important: this zu is not a preposition here. It is part of the infinitive construction.

Because also takes up the first position in the clause, and German main clauses usually put the finite verb in second position.

So the structure is:

1. also
2. müssen
3. wir
4. the rest

That is why you get:

• Also müssen wir ...

not:

• Also wir müssen ...

This is a key German word-order rule called verb-second or V2 word order.

Here also means so or therefore.

It is not the English also meaning too / as well.

So:

• German also = so, therefore
• German auch = also, too, as well

Example:

• Es regnet, also bleiben wir zu Hause. = It’s raining, so we’re staying home.
• Ich komme auch. = I’m coming too.

This is a very common false friend for English speakers.

Ihn is the accusative pronoun for a masculine noun, and it refers back to der Ofen.

• der Ofen = nominative
• den Ofen = accusative
• ihn = him / it (accusative masculine pronoun)

Since vorheizen takes a direct object, the oven becomes the accusative object:

• Wir heizen den Ofen vor.
• Wir heizen ihn vor.

In English we say it, but in German the pronoun must match the noun’s grammatical gender, so Ofen becomes ihn.

In this sentence, nicht negates the action phrase länger vorheizen — in other words, not preheat any longer.

So:

• müssen wir ihn nicht länger vorheizen = we do not need to keep preheating it any longer

German nicht often appears before the part of the sentence it is negating, or toward the end of the clause before the final verb structure.

Here it naturally comes before länger:

• nicht länger = no longer / not any longer

That whole chunk works together.

Länger means longer.

In nicht länger, it means:

• no longer
• not any longer

So ihn nicht länger vorheizen means not preheat it any longer or stop preheating it.

This is slightly more specific than just nicht vorheizen, because it implies the preheating has already been going on for some time.

Yes, vorheizen is a separable verb.

Its prefix is vor-, and in a normal main clause it separates:

• Wir heizen den Ofen vor.

But in an infinitive, it stays together:

• den Ofen vorheizen
• wir müssen den Ofen vorheizen

That is why in your sentence it appears as one word at the end:

• ... müssen wir ihn nicht länger vorheizen.

Because German often sends infinitives to the end of the clause, especially after a modal verb like müssen.

The structure is:

• müssen = finite modal verb
• vorheizen = infinitive

So:

• Wir müssen ihn vorheizen.
• Wir müssen ihn nicht länger vorheizen.

This is one of the most common German sentence patterns:

• Ich will gehen.
• Sie kann kommen.
• Wir müssen warten.

The conjugated modal verb comes earlier, and the main infinitive goes to the end.

Because the subject is wir (we), so the verb must be conjugated for first person plural.

The verb müssen is conjugated like this:

• ich muss
• du musst
• er/sie/es muss
• wir müssen
• ihr müsst
• sie/Sie müssen

Since the subject is wir, the correct form is müssen.

Yes. That would still be completely correct.

• Der Ofen scheint heiß genug zu sein. = The oven seems hot enough.

Adding schon adds the idea of already:

• Der Ofen scheint schon heiß genug zu sein. = The oven seems already hot enough.

So schon gives a little more timing information, but it is not required for the grammar.

Because they are joined by also, and both sides are main clauses.

The sentence is:

1. Der Ofen scheint schon heiß genug zu sein
2. also müssen wir ihn nicht länger vorheizen

Each clause has its own main-clause structure. German does this often when two independent ideas are connected with words like:

• also = so
• aber = but
• und = and
• denn = because

Only subordinate conjunctions like weil, dass, or obwohl would change the word order and push the finite verb later in the clause.