Se a chuva estiver forte, eu vou ficar em casa.

Because after se when you are talking about a possible future condition, Portuguese normally uses the future subjunctive.

• Se a chuva estiver forte... = If the rain is heavy / if it happens to be raining hard
• Se a chuva está forte... sounds wrong in standard Portuguese for this future meaning.
• Está sendo is not natural here. Weather conditions are usually expressed with estar + adjective, not estar sendo.

So this sentence follows a very common pattern:

• Se + future subjunctive, main clause
• Se eu tiver tempo, vou ajudar.
• Se ele chegar cedo, saímos.

Estiver comes from the verb estar.

Here is the relevant comparison:

• infinitive: estar
• present: está
• future subjunctive: estiver

So:

• A chuva está forte. = The rain is heavy.
• Se a chuva estiver forte... = If the rain is heavy...

This is one of the irregular verbs whose future subjunctive form learners usually need to memorize.

Portuguese uses estar because this is a temporary condition, not an inherent characteristic.

• A chuva está forte. = the rain is strong/heavy right now
• forte describes the intensity of the rain at that moment

This matches the normal estar idea of states or conditions.

In weather-related expressions, estar + adjective is very common:

• O tempo está bom.
• O mar está calmo.
• A chuva está fraca.

Because forte is an adjective with the same form for masculine and feminine singular.

So you get:

• o vento forte
• a chuva forte

But in the plural:

• ventos fortes
• chuvas fortes

Many Portuguese adjectives work this way.

Portuguese often uses the definite article more broadly than English does.

So a chuva literally looks like the rain, but in context it may simply correspond to rain in natural English.

Portuguese commonly uses articles with nouns when English might not:

• A chuva está forte.
• O café está quente.
• A vida é curta.

So the article here is completely normal.

Both are possible, but eu vou ficar is much more common in everyday Brazilian Portuguese.

Two ways to express the future here:

• eu vou ficar em casa = very common in speech
• ficarei em casa = correct, but more formal or written

So this sentence sounds very natural and conversational.

Compare:

• Se chover, eu vou ficar em casa.
• Se chover, ficarei em casa.

Both are correct, but the first is what you will hear more often in Brazil.

Ficar em casa means to stay at home.

The verb ficar often means:

• to stay
• to remain
• sometimes to become, depending on context

In this sentence, it clearly means stay:

• vou ficar em casa = I’m going to stay home / I’ll stay at home

Other examples:

• Vou ficar aqui. = I’m going to stay here.
• Ela ficou feliz. = She became happy.

So ficar is a very flexible verb.

Em casa is the normal expression for at home or home when you mean your own home or home in a general sense.

• Estou em casa. = I’m at home.
• Vou ficar em casa. = I’m going to stay home.

Na casa usually means in/at the house of someone or a specific house:

• Estou na casa da minha mãe. = I’m at my mother’s house.
• Eles estão na casa nova. = They are in the new house.

So in your sentence, em casa is the natural choice.

Yes. In Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

So both are natural:

• Se a chuva estiver forte, eu vou ficar em casa.
• Se a chuva estiver forte, vou ficar em casa.

Because vou clearly shows I, the pronoun eu is optional.

People may include eu for:

• emphasis
• contrast
• clarity

For example:

• Se a chuva estiver forte, eu vou ficar em casa, mas eles vão sair.

Yes, absolutely. That is a very common alternative.

Compare:

• Se a chuva estiver forte, eu vou ficar em casa.

  • more literally: If the rain is strong/heavy
  • focuses on the intensity of the rain

• Se chover, eu vou ficar em casa.

  • If it rains
  • focuses on whether rain happens at all

Both are natural, but they are not exactly identical in meaning.

Se means if in this sentence. It introduces a condition.

Structure:

• Se + condition
• result

So:

• Se a chuva estiver forte = If the rain is heavy
• eu vou ficar em casa = I’m going to stay home

This is one of the most important Portuguese sentence patterns.

Other examples:

• Se eu puder, eu vou.
• Se ele ligar, me avise.
• Se vocês chegarem cedo, jantamos juntos.

You can change the order.

Original:

• Se a chuva estiver forte, eu vou ficar em casa.

Also possible:

• Eu vou ficar em casa se a chuva estiver forte.

Both are correct. The version with the se-clause first is very common because it sets up the condition before the result.

When the se-clause comes first, a comma is normally used:

• Se a chuva estiver forte, eu vou ficar em casa.

When it comes second, the comma is often omitted:

• Eu vou ficar em casa se a chuva estiver forte.

A broad, learner-friendly approximation in Brazilian Portuguese would be:

seh a SHOO-va eh-stee-VEHR FOR-chee, eh-o voh fee-KAR ehn KAH-zah

A few helpful notes:

• chuva starts with a sh sound: SHOO-va
• estiver has stress on the last syllable: eh-stee-VEHR
• forte in Brazil is usually pronounced roughly FOR-chee
• vou sounds like voh
• casa often sounds like KAH-zah in Brazilian Portuguese

Pronunciation varies by region, but this is a useful general guide.