El olor del café hace que quiera desayunar, pero primero tengo que destapar la botella de leche.

Because de + el contracts to del in Spanish.

• de el café → del café
• This contraction is required unless el is part of a proper name.

So El olor del café means the smell of the coffee.

Because the subject is specifically the smell of the coffee, not the coffee itself.

• El café hace que... = The coffee makes...
• El olor del café hace que... = The smell of the coffee makes...

Spanish often uses this kind of noun phrase very naturally to identify exactly what is causing the reaction.

It is subjunctive because hacer que is a structure that often triggers the subjunctive when one thing causes another person to feel or do something.

Pattern:

• hacer que + subjunctive

Here:

• El olor del café = the thing causing the effect
• yo = the person affected
• quiera = subjunctive of querer

So the sentence follows the normal pattern:

• algo hace que alguien + subjunctive

Examples:

• La música hace que me sienta mejor.
• El frío hace que cerremos la ventana.

Yes, you could. That is also natural.

There are two common ways to express this idea:

• El olor del café hace que quiera desayunar
• El olor del café me hace querer desayunar

Both are correct, but they are built differently.

• hacer que + subjunctive is a very common and slightly more formal or more structured way
• hacer + infinitive is often more direct

In this sentence, both sound good.

Because after hace que, Spanish normally uses the subjunctive, not the indicative.

So even though the person really does feel like eating breakfast, the grammar is controlled by the expression hacer que:

• correct: hace que quiera
• not here: hace que quiero

This is one of those cases where Spanish grammar depends more on the structure than on the literal certainty of the action.

Because Spanish often leaves out subject pronouns when the verb ending already makes the subject clear.

• quiera can be understood as yo quiera from context
• tengo clearly means I have

Spanish is a pro-drop language, so subject pronouns are often omitted unless needed for emphasis or contrast.

You could say:

• ..., pero primero yo tengo que...

but it would usually sound unnecessary unless you are stressing I.

Desayunar means to have breakfast or to eat breakfast.

It is a verb, so Spanish can say:

• quiero desayunar = I want to have breakfast

Spanish often prefers a verb where English might use a noun phrase.

Compare:

• desayunar = to have breakfast
• almorzar = to have lunch
• cenar = to have dinner

Tener que + infinitive is one of the most common ways to say to have to / must in Spanish.

So:

• tengo que destapar = I have to uncap / open

Pattern:

• tengo que
• tienes que
• tiene que
• tenemos que, etc.

It expresses obligation or necessity.

Both can refer to opening something, but destapar is more specific.

• abrir = to open
• destapar = to uncap, uncover, take the top off

For a bottle, destapar suggests removing the cap or top. That is why it fits well with la botella de leche.

In everyday speech, abrir la botella is also possible, but destapar is more precise here.

Because Spanish often uses the definite article where English may not.

Here la botella de leche refers to a specific bottle that the speaker has in mind, probably the one they are about to use.

Spanish uses articles very frequently with everyday objects:

• cierro la puerta
• cojo el móvil
• abro la botella

Even when English might say open a bottle or just open the milk bottle, Spanish commonly uses the article if the object is identifiable in context.

It means a bottle of milk, that is, a bottle containing milk.

In Spanish, noun + de + noun often expresses content or type:

• una taza de café = a cup of coffee
• una botella de agua = a bottle of water
• una botella de leche = a bottle of milk

So de leche tells you what is inside the bottle.

Primero means first, and it is being used as an adverb to show sequence.

• ..., pero primero tengo que... = ..., but first I have to...

This is a very natural position for it in Spanish. It can sometimes move, but this placement is especially common because it clearly introduces what must happen before the breakfast.

Yes. That is also grammatical.

Compare:

• pero primero tengo que destapar la botella de leche
• pero tengo que destapar primero la botella de leche

Both mean roughly the same thing, but the emphasis shifts slightly:

• primero tengo que... emphasizes first
• destapar primero... puts the sequence marker closer to the action

The original version sounds very natural and conversational.

Because olor is a masculine noun in Spanish, so it takes:

• el olor
• un olor

This has to be learned with the noun. There is not always a reliable logical reason from English, since grammatical gender in Spanish does not always match natural gender or English wording.

Yes, it sounds natural in Spain.

A few notes:

• desayunar is completely standard in Spain
• tengo que is standard everywhere
• destapar la botella is understandable and natural, especially if the focus is on removing the cap

Depending on context, a speaker in Spain might also say:

• abrir la botella de leche
• desenroscar el tapón de la leche if they want to be very specific about unscrewing the cap

But the given sentence is perfectly good Spanish.