Wenn der Backofen heiß genug ist, schiebe ich das Backblech vorsichtig hinein.

Because wenn introduces a subordinate clause. In German, the finite verb normally goes to the end of a subordinate clause.

So:

• der Backofen ist heiß genug = normal main-clause order
• wenn der Backofen heiß genug ist = subordinate-clause order, with ist at the end

This is one of the most important German word-order patterns to learn.

Because the whole wenn-clause takes up the first position in the sentence. In a German main clause, the finite verb must come in second position.

So the structure is:

• Wenn der Backofen heiß genug ist, = position 1
• schiebe = position 2
• ich = comes after the verb

This is often called inversion in English-language German teaching.

Compare:

• Ich schiebe das Backblech vorsichtig hinein.
• Wenn der Backofen heiß genug ist, schiebe ich das Backblech vorsichtig hinein.

It can mean if, when, or even whenever, depending on context.

In this sentence, it could naturally be understood as:

• when the oven is hot enough
• or if the oven is hot enough

German uses wenn for both ideas in many situations.

A useful contrast:

• wenn = if / when for repeated, general, or future situations
• als = when for a single event in the past

So you would not use als here.

In German, genug usually comes after the adjective it modifies.

So:

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

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

Because German nouns have grammatical gender:

• der Backofen = masculine
• das Backblech = neuter

You simply have to learn the noun together with its article.

In this sentence:

• der Backofen is the subject of ist
• das Backblech is the direct object of schiebe

Also note that das looks the same in both nominative and accusative singular, so you do not see a change there.

• der Backofen is nominative, because it is the subject of ist
• das Backblech is accusative, because it is the thing being pushed

So the sentence breaks down like this:

• Wenn der Backofen heiß genug ist
  der Backofen = subject of the clause

• schiebe ich das Backblech vorsichtig hinein
  ich = subject
  das Backblech = direct object

This is a good example of how each clause has its own structure.

Schieben means to push or to slide/push something along a surface.

Here it is used naturally for putting a baking tray into an oven, because you are typically sliding it in rather than simply placing it.

So schiebe ich das Backblech hinein feels more specific than a general verb like:

• legen = lay/place
• stellen = stand/set upright
• tun = put (very general, often less elegant)

In cooking instructions, schieben is very common for trays, racks, and oven dishes.

Hinein means in / into it / inside, with a sense of movement into something.

In this sentence, it shows the direction of the movement: the tray is being pushed inside the oven.

It appears at the end because German often places directional adverbs or particles there quite naturally.

So:

• hinein = into, inwards
• schiebe ... hinein = push/slide ... in

You can think of it as adding the idea of into the inside.

The difference is about direction relative to the speaker:

• hinein = into there, away from the speaker’s point of view
• herein = into here, toward the speaker’s point of view

In an oven sentence, hinein is the natural choice, because you are pushing the tray into the oven, not toward where the speaker is standing.

This same pattern appears in other word pairs:

• hinauf / herauf
• hinunter / herunter
• hinein / herein

Yes. German often allows both patterns:

• Ich schiebe das Backblech vorsichtig in den Backofen.
• Ich schiebe das Backblech vorsichtig hinein.
• Ich schiebe das Backblech vorsichtig in den Backofen hinein.

All of these are possible.

The version with only hinein works because the destination is already obvious from context: you are talking about the oven in the first clause.

The version with in den Backofen is more explicit.

Vorsichtig is an adverb meaning carefully. It describes how the tray is pushed in.

German adverbs are somewhat flexible, but this placement is very natural:

• schiebe ich das Backblech vorsichtig hinein

A rough pattern here is:

1. verb
2. subject
3. object
4. manner adverb
5. direction

So:

• schiebe = verb
• ich = subject
• das Backblech = object
• vorsichtig = how?
• hinein = where to?

Other placements may be possible in certain contexts, but this is a standard, natural one.

Yes. You could say:

• Wenn der Backofen heiß genug ist, dann schiebe ich das Backblech vorsichtig hinein.

The dann is optional here.

Adding dann can make the result feel a bit more explicit: when/if that condition is met, then I do this.

Without dann, the sentence is still completely correct and very natural. In everyday German, speakers often leave it out.