Learn Portuguese (Portugal): Quando o pai se acalma, o bebé está prestes a nascer. - When the father calms down, the baby is about to be born.

Both expressions indicate an imminent action. However, estar prestes a followed by an infinitive like nascer is stronger, meaning something is on the very brink of happening right now. estar para can sometimes be slightly less urgent or just indicate an intention. Because birth is an immediate event here, está prestes a is the perfect fit.

I learned that quando triggers the future subjunctive. Why is it the present tense se acalma here?

You use the future subjunctive (like Quando o pai se acalmar...) when talking about a specific event that hasn't happened yet. Because this sentence uses the present indicative (se acalma), it reads either as a narrative describing a scene happening right now in a story, or as a general statement meaning 'Whenever the father calms down'.

Why is the reflexive pronoun se placed before the verb in se acalma?

In European Portuguese, pronouns normally follow the verb (O pai acalma-se). However, the conjunction Quando acts as a 'magnet' word. It pulls the pronoun to the position before the verb, which is a rule you will often see with other magnet words like que or porque.

Is there a reason bebé is spelled with an acute accent instead of a circumflex?