Learn Hardcore Catalan: Ahir vaig caure al carrer i ara tinc una ferida a la cama. - Yesterday I fell in the street and now I have a wound on my leg.

Breakdown of Ahir vaig caure al carrer i ara tinc una ferida a la cama.

Why is vaig caure used here? I thought vaig meant I go.

In this sentence, vaig caure is the periphrastic past, one of the most common ways to say the past in Catalan.

This is different from the present-tense meaning of vaig on its own:

So the infinitive caure is what tells you this is a past-tense structure, not the verb to go.

Why does the sentence use ahir vaig caure but ara tinc? Why two different tenses?

Because the sentence talks about two different time frames:

ahir vaig caure = something that happened yesterday, so Catalan uses a past tense
ara tinc una ferida = something that is true now, so Catalan uses the present

This is very natural in Catalan, just like in English:

What exactly is ahir doing, and does it have to go at the beginning?