Ontem, o alarme não tocou e perdemos os primeiros segundos da aula.

Yesterday, the alarm did not ring and we missed the first seconds of the class.

Breakdown of Ontem, o alarme não tocou e perdemos os primeiros segundos da aula.

• de: of
• e: and
• ontem: yesterday
• não: not

Questions & Answers about Ontem, o alarme não tocou e perdemos os primeiros segundos da aula.

Why is ontem placed at the beginning of the sentence and followed by a comma?

Putting ontem (“yesterday”) at the start is a way to highlight the time frame. Portuguese allows adverbs of time to appear at the front for emphasis or style. The comma after ontem isn’t mandatory, but it signals a slight pause and makes the sentence easier to read. You could also write it without a comma:
Ontem o alarme não tocou e perdemos os primeiros segundos da aula.

Why does the sentence use o alarme instead of um alarme?

Using the definite article o (“the”) suggests you’re referring to a specific alarm that both speaker and listener know about—probably the school’s alarm. If you said um alarme, it would sound like you introduced a new, unspecified alarm into the conversation.

In English we say “the alarm didn’t go off.” Why does Portuguese use tocar and não tocou?

Portuguese uses tocar (literally “to play” or “to ring”) for alarms, bells and phone ringtones. So “the alarm rang” = o alarme tocou. To negate, you place não before the verb: o alarme não tocou (“the alarm didn’t ring”).

Could you use soar instead of tocar for the alarm?