Learn Hardcore Portuguese (Portugal): Se começar uma tempestade, vamos ficar em casa.

In this sentence, se is the conjunction if, introducing a conditional clause. It has nothing to do with the reflexive pronoun se; here it corresponds directly to English if.

Why is começar in the infinitive and not conjugated as começa or comece? And why does uma tempestade come after the verb?

Portuguese often keeps the verb in the infinitive when the subject follows it—a stylistic or more impersonal choice. By postponing the subject (uma tempestade), we leave the verb uninflected. If you move the subject in front, you must conjugate the verb:
• Se a tempestade começar… (subjunctive, common in European Portuguese)
• Se a tempestade começa… (indicative, more common in Brazilian Portuguese)

Can I use the present indicative or present subjunctive instead of the infinitive in the se-clause?

Yes. For real, likely conditions:
• European Portuguese often uses the present subjunctive: Se a tempestade comece, …
• Brazilian Portuguese often uses the present indicative: Se a tempestade começa, …
The infinitive version with subject postponed sounds more formal or literary.

Why do we say vamos ficar instead of ficaremos? Are they interchangeable?

Both express the future:
• Ficaremos is the simple future (“we will stay”).
• Vamos ficar is the periphrastic future or “near future” (“we’re going to stay”).
In everyday European Portuguese, vamos ficar is more common for immediate or planned actions, while ficaremos is more formal.

Why is it em casa and not na casa or numa casa?

The fixed expression ficar em casa means stay at home. We drop the article before casa when it’s used in this locative sense.
• To say in a (random) house, use numa casa.
• To specify in our house, say na nossa casa.

Why do we use uma tempestade (a storm) instead of a tempestade (the storm)?

Using uma makes the storm indefinite, meaning any storm might start. If you have a specific storm in mind, you’d use the definite article: Se a tempestade começar...

Can I flip the order and say Vamos ficar em casa, se começar uma tempestade? And is the comma still needed?

Yes, you can place the main clause first. When the se-clause comes after the main clause, the comma before se is optional, depending on how strong a pause you want. If the se-clause comes before the main clause, the comma is mandatory.

Why is the pronoun nós omitted before vamos? Can I say Nós vamos ficar em casa?

Portuguese is a pro-drop language: subject pronouns are often omitted because the verb ending shows the subject. You can say Nós vamos ficar em casa to add emphasis or clarity, but it’s not necessary.