Wenn du den Ofen nicht vorheizt, scheint der Kuchen länger zu brauchen.

Because wenn introduces a subordinate clause. In German subordinate clauses send the conjugated verb to the end.

So:

• Wenn du den Ofen nicht vorheizt = subordinate clause
• the verb vorheizt goes to the end

Compare:

• Main clause: Du heizt den Ofen nicht vor.
• Subordinate clause: ..., wenn du den Ofen nicht vorheizt.

With a separable verb like vorheizen, the prefix vor- stays attached when the verb is at the end of a subordinate clause.

Because German main clauses follow the verb-second rule.

The whole wenn-clause takes the first position in the sentence:

• Wenn du den Ofen nicht vorheizt, = position 1

So the finite verb of the main clause must come next:

• scheint = position 2

Then the subject comes after it:

• der Kuchen

That is why you get:

• Wenn du den Ofen nicht vorheizt, scheint der Kuchen länger zu brauchen.

You could also start with the main clause:

• Der Kuchen scheint länger zu brauchen, wenn du den Ofen nicht vorheizt.

Because Ofen is the direct object of vorheizen, so it has to be in the accusative case.

Ofen is masculine:

• nominative: der Ofen
• accusative: den Ofen

So:

• du vorheizt den Ofen = you preheat the oven

Because the verb stem ends in -z.

For du forms, German normally adds -st, but after stems ending in sounds like s, ß, x, z, German usually does not add an extra s sound.

So:

• heizen → du heizt
• vorheizen → du vorheizt

This is similar to:

• heißen → du heißt

So vorheizt is the correct form here.

Yes. Vorheizen is a separable verb.

In a normal main clause, the prefix splits off:

• Du heizt den Ofen vor.

In a subordinate clause or with an infinitive, it stays together:

• ..., wenn du den Ofen vorheizt
• den Ofen vorzuheizen

So in your sentence, vorheizt stays as one word because it is at the end of a subordinate clause.

Because German normally uses a comma to separate a subordinate clause from a main clause.

Here:

• Wenn du den Ofen nicht vorheizt, = subordinate clause
• scheint der Kuchen länger zu brauchen. = main clause

That comma is required in standard German.

Scheinen + zu + infinitive means to seem to ...

So:

• Der Kuchen braucht länger. = The cake takes longer.
• Der Kuchen scheint länger zu brauchen. = The cake seems to take longer.

Using scheint makes the statement less direct and more observational. It suggests the speaker is judging from experience or appearance, not stating it as a hard fact.

Structure:

• scheint = conjugated verb
• zu brauchen = infinitive construction

Because German often uses the real subject directly with scheinen.

English often says:

• It seems the cake takes longer.

German more naturally says:

• Der Kuchen scheint länger zu brauchen.

So instead of a dummy it, German uses der Kuchen as the subject of scheint.

Usually, yes: brauchen often means to need.

But in everyday German it can also mean to take in the sense of taking time.

So here:

• Der Kuchen braucht länger.

means something like:

• The cake takes longer.

This use is very common in spoken and informal written German.

Länger is the comparative form of lang and here it works adverbially, meaning for a longer time or longer.

So:

• lange = long / for a long time
• länger = longer

In this sentence it modifies the idea of brauchen:

• länger brauchen = to take longer

This is a very common expression in German.

Yes, it is natural.

A very straightforward version would be:

• Wenn du den Ofen nicht vorheizt, braucht der Kuchen länger.

Your sentence with scheint is also natural, but slightly softer and more tentative:

• Wenn du den Ofen nicht vorheizt, scheint der Kuchen länger zu brauchen.

That sounds like the speaker is drawing a conclusion from observation rather than making a flat statement.