Se o espinafre estiver fresco, eu vou cozinhar arroz para o jantar.

Because after se when Portuguese talks about a future possibility or condition, it usually uses the future subjunctive.

So:

• Se o espinafre estiver fresco... = If the spinach is fresh... / If the spinach turns out to be fresh...

Using está here would sound wrong for this future-condition meaning. In English, we use the present after if, but in Portuguese, this kind of sentence normally calls for the future subjunctive.

Estiver is the future subjunctive form of estar.

Here is the pattern:

• se eu estiver
• se você/ele/ela estiver
• se nós estivermos
• se vocês/eles/elas estiverem

This tense is very common after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = as soon as

when the sentence refers to the future.

Because the speaker is talking about something that is not confirmed yet. The spinach may or may not be fresh. That uncertainty is exactly the kind of situation where Portuguese often uses the subjunctive.

Compare:

• Se o espinafre estiver fresco, eu vou cozinhar arroz.
  Future possibility.

• Se o espinafre estivesse fresco, eu cozinharia arroz.
  Hypothetical/unreal situation.

So the subjunctive helps show that this is a condition, not a fact.

In Brazilian Portuguese, ir + infinitive is extremely common for talking about the future.

So:

• eu vou cozinhar = I’m going to cook / I will cook

This is often more natural in everyday speech than:

• eu cozinharei

The simple future exists, but in conversation it can sound more formal, literary, or less common depending on context.

Yes. Portuguese often drops subject pronouns when the meaning is clear.

So these both work:

• Se o espinafre estiver fresco, eu vou cozinhar arroz para o jantar.
• Se o espinafre estiver fresco, vou cozinhar arroz para o jantar.

Including eu can add emphasis, contrast, or clarity, but it is not necessary here.

Because espinafre is a masculine singular noun in Portuguese.

So:

• o espinafre fresco

Even though spinach is often thought of as an uncountable food in English, Portuguese still gives it grammatical gender, and that gender is masculine.

Because adjectives in Portuguese usually agree with the noun in gender and number.

Here:

• espinafre is masculine singular
• so the adjective must also be masculine singular
• therefore: fresco

Examples:

• o espinafre fresco
• a alface fresca
• os legumes frescos
• as verduras frescas

Because arroz here is being used in a general, indefinite way: cook rice.

So:

• cozinhar arroz = cook rice

If you say cozinhar o arroz, it usually sounds more like cook the rice, meaning a specific rice that has already been mentioned or is understood from context.

Both are possible, but they mean slightly different things.

Para o jantar means for dinner.

It tells you the purpose of the rice:

• vou cozinhar arroz para o jantar = I’m going to cook rice for dinner

It does not necessarily mean at dinner in a time sense. It means the rice is intended to be part of dinner.

Yes. Portuguese can also put the main clause first:

• Eu vou cozinhar arroz para o jantar se o espinafre estiver fresco.

This means the same thing. The version with se... first may sound a little more natural when you want to highlight the condition first.

Yes, but it changes the tone slightly.

• Se o espinafre estiver fresco... = If the spinach is fresh...
• Caso o espinafre esteja fresco... = In case the spinach is fresh... / If the spinach happens to be fresh...

Caso is a bit more formal or structured. Also notice that with caso, Portuguese normally uses the present subjunctive:

• caso esteja

while with se in a future condition, it uses the future subjunctive:

• se estiver

Yes, it is natural and correct. Cozinhar arroz means to cook rice.

Depending on context, Brazilians might also say things like:

• fazer arroz = make rice
• preparar arroz = prepare rice

But cozinhar arroz is completely normal and easy to understand.

It is neutral and very natural. Nothing in it sounds overly formal or overly slangy.

• Se o espinafre estiver fresco = grammatically standard and common
• eu vou cozinhar = very natural in everyday Brazilian Portuguese
• para o jantar = neutral and common

So this is a good sentence model for normal, standard Brazilian Portuguese.