Se non scongelo il pane in tempo, rimetto tutto nel congelatore.

Italian often uses the definite article with food and other mass nouns where English would often leave it out.

So il pane can simply mean bread in general, or the bread being talked about in that situation.

This is very normal in Italian:

• Mangio il pane
• Compro il latte
• Metto il burro in frigo

So in this sentence, il pane is not unusual at all.

Scongelo is the first person singular present indicative of scongelare.

It means:

• I defrost
• I thaw out

It is related to congelare, which means to freeze. The prefix s- often gives the idea of reversing an action, so scongelare is basically to unfreeze / defrost.

Because Italian often uses the present indicative in both parts of a real or likely se sentence.

So:

Se non scongelo il pane in tempo, rimetto tutto nel congelatore

can mean either:

• a habitual action: Whenever I do not defrost the bread in time, I put everything back in the freezer
• a future result: If I do not defrost the bread in time, I will put everything back in the freezer

English often uses will in the second part, but Italian commonly does not need it here.

Because this is a real / possible condition, and after se Italian normally uses the indicative, not the subjunctive.

So this is correct:

• Se non scongelo il pane in tempo, rimetto tutto nel congelatore

For an unreal or hypothetical condition, Italian uses a different pattern:

• Se non scongelassi il pane in tempo, rimetterei tutto nel congelatore

That means something more like If I did not defrost the bread in time, I would put everything back in the freezer.

Rimetto comes from rimettere, which means to put back.

It is built from:

• mettere = to put
• ri- = again / back

So rimettere literally means to put again / put back.

Its present tense is irregular because mettere is irregular:

• metto = I put
• rimetto = I put back

Tutto means everything or all of it.

In context, it probably refers to:

• all the bread
• or all the food/items that were taken out

If the speaker only wants to refer clearly to the bread, they could also say:

• Se non scongelo il pane in tempo, lo rimetto nel congelatore

That would mean I put it back in the freezer and sounds more specific.

In tempo means in time, in the sense of before it is too late.

So scongelare il pane in tempo means to defrost the bread soon enough.

A useful contrast is:

• in tempo = in time
• per tempo = early enough / with enough notice

In this sentence, in tempo is the natural choice because the idea is before it is too late to use it.

Because nel is the contraction of in + il.

So:

• in + il congelatore → nel congelatore

This is very common in Italian:

• nel frigo
• nel forno
• nel cassetto

Also, congelatore means freezer. It can refer to:

• a standalone freezer
• or the freezer compartment of a fridge

It can describe both, depending on context.

It may mean:

• a habit: Whenever I do not defrost the bread in time, I put everything back in the freezer
• a future situation: If I do not defrost the bread in time, I will put everything back in the freezer

Italian often leaves that distinction to the context.

It is very natural and standard to put a comma when the se clause comes first:

Se non scongelo il pane in tempo, rimetto tutto nel congelatore.

In short sentences, some writers may omit it, but the comma is common and helpful.

If the main clause comes first, the comma is often omitted:

• Rimetto tutto nel congelatore se non scongelo il pane in tempo.

Yes, but rimetto is more natural and compact here.

Compare:

• rimetto tutto nel congelatore = I put everything back in the freezer
• metto di nuovo tutto nel congelatore = I put everything in the freezer again

The second version is understandable, but rimettere is the usual verb when something goes back to where it was before.

Yes. It can be used both transitively and intransitively, depending on the context.

Examples:

• Scongelo il pane = I defrost the bread
• Il pane si scongela in fretta = The bread defrosts quickly

In your sentence it is transitive, because the speaker is actively defrosting il pane.