Learn Spanish (Spain): Compré un sobre blanco.

Compro is the present tense (I buy), but here we are talking about a completed action in the past (I bought). For regular -ar verbs like comprar, you drop the -ar ending and add -é to make the yo (I) form in the preterite tense.

Is the accent mark on compré important?

Yes, it is very important! It tells you to stress the final syllable (com-PRÉ). Without the written accent, it would be a completely different tense, like a command. The yo and él/ella/usted forms of regular verbs in this past tense always need their accent marks to be understood correctly.

I thought sobre meant 'on' or 'about'. Why does it mean 'envelope' here?

You are right that sobre is often a preposition meaning 'on' or 'about'. However, Spanish has words with multiple meanings. Because it comes right after un (a/an) here, it is acting as a noun. The noun el sobre means 'the envelope'.

Why does blanco come after sobre instead of before it?

In Spanish, descriptive adjectives like colors almost always come after the noun they describe. Instead of saying 'a white envelope' like we do in English, the structure is 'an envelope white' (un sobre blanco).