The letter 'A' / sentence structure

Can you please explain why in some sentences the letter ‘A’ appears to signify the word ‘in’

e.g. O autocarro vai chegar daqui a doze minutos - The bus is going to arrive in twelve minutes.

Thanks in advance.

It means “at” also. Same in French

In that case, the word “a” is being used as a preposition, and like other Portuguese prepositions, they might need to be translated in several different ways into English, depending on the sentence. I feel that for every three or four English prepositions, Portuguese might require only one or two!

Apart from its use as a preposition, the word “a” may also be a feminine definite article (the).

If you tried to translate that sentence more literally, you could think of it as something like “The bus is going to arrive from here to twelve minutes” (meaning… within the time frame from now until 12 minutes from now.)

In English we word this same concept differently. We say “…to arrive in twelve minutes”. Like Joseph said, when it comes to words like this that serve a grammatical function, like prepositions, there is rarely a one-to-one translation.

This unit might help, especially the Learning Note titled The Preposition A (to see some other examples of how it can be translated differently):

Prepositions 1