Absolute Infinitive

When used to express a condition, the a + infinitive construction is grammatically classified as an oração subordinada adverbial condicional reduzida de infinitivo (a conditional reduced infinitive clause). While it is a hallmark of formal and literary Portuguese in Brazil, outside of lexicalized phrases, this conditional construction is much more common in everyday spoken and written European Portuguese.

1. Structure and Agreement:
It is formed using the preposition a followed by the infinitive. To form the negative, place não between the preposition and the verb (e.g., 'A não ser verdade...' - If it is not true...). Note that a não ser has also lexicalized into a highly frequent fixed expression meaning 'except' or 'unless'. It is crucial to distinguish between the reduced clause a não ser (which takes the infinitive) and the full conjunction a não ser que (which requires a conjugated subjunctive verb). Additionally, a não ser can function simply as a preposition of exception with nouns or pronouns (e.g., 'Todos concordaram, a não ser o João' - Everyone agreed, except João), which is distinct from its conditional use. Regarding subject agreement, the baseline 'Same Subject' rule applies: if the subject of the conditional clause is exactly the same as the subject of the main clause, the impersonal (uninflected) infinitive is typically used (e.g., 'A sair cedo, chego a tempo' - If I leave early, I'll arrive on time). However, if the subjects are different, or if the subject of the reduced clause is explicitly plural or refers to 'we/you/they', you must use the personal infinitive (e.g., 'A serem verdadeiros os boatos...' - If the rumors are true...). Additionally, when the subject is explicitly stated in these clauses, it almost always follows the verb (Subject-Verb Inversion). This applies to nouns as well as personal pronouns (e.g., 'A sermos nós a decidir...' - If it were up to us to decide...). This inversion is the standard syntactic word order for this specific conditional construction.

2. The Golden Rule (Equivalence to 'Se'):
The easiest way to become confident with this structure is to remember that it generally equals se + subjunctive. Crucially, the tense equivalence depends entirely on the context and the tense of the main clause. For example, the exact same phrase a ser verdade can equate to the Future Subjunctive for present/future conditions ('A ser verdade, ficarei feliz' = 'Se for verdade...'), or the Imperfect Subjunctive for past counterfactuals ('A ser verdade, eu teria ficado feliz' = 'Se fosse verdade...'). While this pattern is mostly restricted to fixed phrases in Brazilian Portuguese (such as with ser, julgar, dizer, and crer), it is highly productive in European Portuguese, where it can be used with almost any verb.

3. Function and Nuance:
It typically replaces a full subordinate clause to express:

• Condition: 'A crer no que dizem...' (If we are to believe what they say...) or 'A ser verdade o que dizes, não te perdoo' (If what you say is true, I do not forgive you).
• Cause/Judgment: 'A julgar pelo barulho...' (Judging by the noise...).
• Discourse Marker/Perspective: While the primary function is conditional, some phrases like 'A dizer a verdade...' (To tell the truth...) have lexicalized into discourse markers expressing perspective, and do not strictly follow the 'se + subjunctive' equivalence.

4. Distinctions from Other 'A + Infinitive' Uses:
Do not confuse this conditional structure with other common uses:

• Time vs. Condition: Do not confuse the conditional a + infinitive with the temporal ao + infinitivo construction. For example, a chegar means 'If [subject] arrives', whereas ao chegar means 'Upon arriving' or 'When [subject] arrives'.
• Progressive (PT-PT): The estar a + infinitivo structure is used for ongoing actions in European Portuguese (e.g., estou a ler - I am reading).
• Prepositional Verbs: Many verbs naturally require the preposition a before a following infinitive (e.g., começar a chorar, aprender a ler).
• Imperative: Used as a command (e.g., A trabalhar! - Get to work!).

5. Punctuation:
As a subordinate adverbial clause, it requires a comma when placed at the beginning of the sentence (ordem inversa). However, when placed at the end of the sentence, the comma is often optional.

6. Alternative with 'De':
For advanced learners, note that the preposition de can also be used as an alternative to a to form conditional reduced infinitives in formal Portuguese (e.g., 'De saber isso, não teria vindo' - Had I known that, I wouldn't have come).