Learn Hardcore Portuguese (Portugal): Se o saldo estiver baixo, a transferência pode falhar. - If the balance is low, the transfer may fail.

Portuguese uses the subjunctive after se to express an uncertain or future condition. Here, the condition (the balance being low at the time of the transfer) is hypothetical/forward-looking, so you use the subjunctive: Se o saldo estiver baixo... Using está would suggest a known, current fact, not a hypothetical condition.

What tense/mood is estiver, and when do I use it?

It’s the future subjunctive (futuro do conjuntivo), which Portuguese uses after time/condition words like se, quando, assim que, logo que when referring to future or uncertain events.

Future subjunctive of estar: eu/ele estiver, tu estiveres, nós estivermos, vós estiverdes, eles estiverem.
Compare: present subjunctive (used with triggers like caso, talvez): eu/ele esteja, tu estejas, etc.

Can I say Se o saldo está baixo?

In careful/standard European Portuguese, prefer Se o saldo estiver baixo for a hypothetical condition. You may hear the indicative (está) in casual speech when the condition is treated as a known present fact, but for neutral, correct usage in writing or formal speech, use the future subjunctive (estiver) after se in conditional contexts.

Can I use caso instead of se?

Yes, but the verb form changes: Caso o saldo esteja baixo, a transferência pode falhar. After caso, Portuguese uses the present subjunctive (esteja), not the future subjunctive.

Could I say Se o saldo for baixo (with ser)?

That sounds odd because baixo here describes a temporary state, which pairs with estar. More idiomatic alternatives are:

Se o saldo estiver baixo, ... (state)
Se o saldo for insuficiente, ... (quality/judgment; ser works well with adjectives like insuficiente)
Se houver pouco saldo, ... (there is little balance)

Should it be abaixo instead of baixo?

Use baixo as an adjective (low). Abaixo is mainly an adverb meaning “below.” You’d use abaixo with a threshold: Se o saldo estiver abaixo de 20 €, a transferência pode falhar. Otherwise, baixo is the right choice: o saldo estiver baixo.

Why is it baixo and not baixa?

Agreement: saldo is masculine singular, so the adjective is masculine singular baixo. If the noun were feminine (e.g., a conta), you’d use baixa to agree with that noun.

Do I need the definite article o in o saldo?

Yes. In European Portuguese, singular count nouns typically take the definite article when referring to a specific, context-known thing, even if English might omit “the.” You could also make ownership explicit:

o seu saldo (formal/neutral “your” balance)
o teu saldo (informal “your” in Portugal)
o saldo da conta (the account’s balance)

What exactly does saldo mean here?

It means the account balance (amount of money available). Note: the plural saldos (in shops) means seasonal sales/clearance—unrelated to banking.

What does transferência mean, and how is it used?

Here it means a bank transfer. Common collocations:

transferência bancária (bank transfer)
transferência de dinheiro (transfer of money)
transferência para a conta X (transfer to account X) Contrast with:
pagamento (a payment, not necessarily a transfer)
transação (any transaction, broader term)

What’s the nuance of pode falhar? Could I say poderá falhar or rephrase it?

pode falhar = may/can fail (general possibility; neutral and common).
poderá falhar = might/will possibly fail (often more formal/speculative, sometimes suggesting a future prediction). Natural rephrasings:
A transferência pode não ser efetuada.
A transferência pode ser recusada.
A operação pode dar erro. (informal)

Why is there a comma, and can I move the clauses around?

When the conditional clause comes first, Portuguese uses a comma: Se o saldo estiver baixo, a transferência pode falhar. If you invert the order, you normally drop the comma: A transferência pode falhar se o saldo estiver baixo. Both are fine.

How do I pronounce the sentence in European Portuguese?

Approximate IPA: [sɨ u ˈsaɫdu ʃtiˈvɛɾ ˈbajʃu, ɐ tɾɐ̃ʃfɨˈɾẽsjɐ ˈpɔðɨ fɐˈʎaɾ]

Tips:

estiver: initial “es-” sounds like “sh” before “t” → [ʃt-].
lh in falhar is a palatal sound [ʎ], like the “lli” in “million.”
Final unstressed -e in pode is a reduced vowel [ɨ].
r at the end of falhar is weak in many accents.

Is falhar transitive here? Do I need a preposition?

Here falhar is intransitive: a transferência pode falhar (“the transfer may fail”). With activities, you’ll see falhar em (“fail at”): falhar em pagar. Falhar can also be transitive in other senses: falhou o penálti (“he missed the penalty”).

Is the sentence the same in Brazil?

Yes, it’s fully natural in Brazil too: Se o saldo estiver baixo, a transferência pode falhar. You’ll also hear alternatives like A transferência pode não ser concluída or A operação pode dar errado (informal).

Could I use quando instead of se?

Only if you mean “whenever/when” (a time that is expected to occur), not a mere possibility. For example:

Hypothetical: Se o saldo estiver baixo, ... (if)
Eventual/whenever: Quando o saldo estiver baixo, ... (when/whenever)