Antaŭ ol mi decidas kion kuiri, mi malfermas la fridujon kaj rigardas kio restas en ĝi.

Antaŭ ol means before in the sense of before some action happens.

• antaŭ = before
• ol = than / in relation to a comparison, and it is also used after antaŭ in this pattern

So:

• Antaŭ ol mi decidas... = Before I decide...

This is a very common Esperanto structure. English uses just before, but Esperanto usually uses antaŭ ol when a whole clause follows.

Examples:

• Antaŭ ol mi foriras, mi fermas la fenestron. = Before I leave, I close the window.
• Antaŭ ol manĝi, li lavas siajn manojn. = Before eating / before he eats, he washes his hands.

The whole sentence is in the present tense, describing something that happens generally, habitually, or as a vivid present-time sequence.

So:

• mi decidas = I decide
• mi malfermas = I open
• mi rigardas = I look

The idea is:

Before I decide what to cook, I open the fridge and look at what is left in it.

This is not necessarily talking about one specific past event. It sounds like a routine or usual process.

If you wanted past time, you could say:

• Antaŭ ol mi decidis kion kuiri, mi malfermis la fridujon...

If you wanted future time:

• Antaŭ ol mi decidos kion kuiri, mi malfermos la fridujon...

Because kion is the accusative form of kio.

Here, kion is the object of kuiri:

• kuiri ion = to cook something
• kion kuiri = what to cook

So the hidden relationship is:

• I cook something
• what = kion

That is why -n is needed.

Compare:

• Mi scias kion vi faras. = I know what you are doing.
• Mi demandas kion li volas. = I ask what he wants.

If you said kio kuiri, that would be ungrammatical here, because kio would not be marked as the direct object of kuiri.

In Esperanto, the infinitive already includes the idea of English to + verb.

So:

• kuiri = to cook
• manĝi = to eat
• iri = to go

That means:

• kion kuiri = what to cook
• kien iri = where to go
• kiam veni = when to come

You do not add another word for to before the infinitive.

It is an indirect question or embedded question.

A direct question would be:

• Kion mi kuiru? = What should I cook?

Inside a larger sentence, Esperanto often uses the same ki- words:

• Mi decidas kion kuiri. = I decide what to cook.
• Mi ne scias kie li estas. = I don’t know where he is.
• Ŝi demandas kiam ni alvenos. = She asks when we will arrive.

So kion kuiri is not a separate spoken question here, but it still has question-word structure.

Because la fridujon is the direct object of malfermas.

• malfermi = to open
• You open something
• That something takes -n

So:

• Mi malfermas la fridujon. = I open the fridge.

Without -n, it would be incorrect in normal Esperanto grammar.

Also:

• fridujo literally means a cold-container, and in normal usage it means fridge / refrigerator.

Because inside that second clause, kio is the subject of restas, not the object.

Think of the meaning:

• what remains in it
• The thing itself is the thing that remains

So:

• kio restas = what remains / what is left

Here, kio is doing the action of resti in a grammatical sense, so it stays nominative.

Compare the contrast:

• kion kuiri = what to cook
  Here what is the object of cook, so it becomes kion.

• kio restas = what remains
  Here what is the subject of remains, so it stays kio.

This is one of the most important things to notice in the sentence.

Resti means to remain, to stay, or to be left.

In this sentence:

• kio restas en ĝi = what is left in it / what remains in it

So the speaker opens the fridge and checks what food is still there.

Some related examples:

• Restas pano sur la tablo. = Bread remains on the table. / There is bread left on the table.
• Kiom da tempo restas? = How much time is left?
• Mi restas hejme. = I stay at home.

Ĝi is the pronoun it.

Here it refers back to la fridujo:

• mi malfermas la fridujon kaj rigardas kio restas en ĝi
• I open the fridge and look at what is left in it

So:

• en = in
• ĝi = it
• en ĝi = in it

Esperanto often uses pronouns very straightforwardly like this.

In Esperanto, rigardi usually takes a direct object without a preposition.

So:

• rigardi ion = to look at something

That is why:

• mi rigardas kio restas en ĝi is normal Esperanto

In English, look usually needs at, but Esperanto verbs do not always match English preposition patterns.

Compare:

• Mi rigardas la ĉielon. = I look at the sky.
• Li rigardis min. = He looked at me.

This is a good reminder not to translate prepositions too literally from English.

Esperanto word order is fairly flexible, but this version is the most natural and clear.

Current sentence:

• Antaŭ ol mi decidas kion kuiri, mi malfermas la fridujon kaj rigardas kio restas en ĝi.

You could change parts for emphasis, but not every rearrangement sounds equally natural.

For example, these are still understandable:

• Mi malfermas la fridujon kaj rigardas kio restas en ĝi antaŭ ol mi decidas kion kuiri.
• Antaŭ ol decidi kion kuiri, mi malfermas la fridujon...

Because Esperanto marks objects with -n, it allows more freedom than English. But learners should usually stick to the more standard order until they feel comfortable.

Yes. A very natural alternative is:

• Antaŭ ol decidi kion kuiri, mi malfermas la fridujon...

That means:

• Before deciding what to cook, I open the fridge...

Both are correct.

The difference is mostly stylistic:

• Antaŭ ol mi decidas... explicitly includes the subject I
• Antaŭ ol decidi... is a little more compact

Esperanto often allows both a full clause and an infinitive phrase in this kind of situation.

No. Fridujo is common and easy to understand, but you may also see fridilo.

The difference comes from Esperanto word-building:

• -ujo = container / holder
• -ilo = tool / instrument

So:

• fridujo = cold-container
• fridilo = cooling device

In real usage, both can refer to a refrigerator. Many speakers use fridujo in everyday speech. A learner should recognize both. In your sentence, fridujo is completely normal.