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

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

• vaig
  • infinitive = I did / I ...-ed
• so vaig caure = I fell

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

• vaig = I go
• vaig caure = I fell

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

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:

• Yesterday I fell...
• ...and now I have a wound...

Ahir means yesterday. It sets the time for vaig caure.

It does not have to be at the beginning, although that is a very common position.

You could also say:

• Vaig caure ahir al carrer...

Putting ahir first is often slightly more natural when you want to frame the whole first event in time right away.

Because al is the contraction of a + el.

• a = to / at / in / on
• el = the
• a + el = al

So:

• al carrer = in/on the street

This contraction is required in standard Catalan.

Similar contractions:

• del = de + el
• pel = per + el

In this sentence, al carrer means the place where the fall happened: in the street / on the street.

Catalan uses a in many location expressions where English may choose in, on, or at depending on context. So you should not expect a one-to-one match with English prepositions.

Here, caure al carrer is a normal way to say that someone fell while in the street.

Catalan often uses tenir (to have) for physical conditions, injuries, symptoms, and similar things.

So:

• tinc una ferida = I have a wound
• tinc febre = I have a fever
• tinc mal de cap = I have a headache

You can use estar in some related situations, but the meaning shifts:

• estic ferit / ferida = I am injured

That focuses more on your state.
Tinc una ferida focuses on the wound itself.

Ferida is a feminine singular noun.

So it takes:

• una = feminine singular indefinite article

That is why you get:

• una ferida

If it were masculine, you would expect un instead.

Related forms:

• ferida = wound, injury
• ferit = injured (masculine adjective or past participle)
• ferida = injured (feminine adjective) as well, depending on context

So ferida can be a noun here, but it also exists as an adjective in other sentences.

Catalan often uses the definite article with body parts where English often uses a possessive.

So Catalan prefers:

• una ferida a la cama
• literally: a wound on the leg

Even though in English we usually say:

• a wound on my leg

The owner is understood from context, so la is enough.

This is very common in Catalan:

• Em fa mal el cap = My head hurts
• Tinc dolor a l’esquena = I have pain in my back

Catalan often uses a to mark the location of something on the body.

So:

• una ferida a la cama
• mal al braç
• un cop al cap

English might use on, in, or sometimes no preposition at all depending on the phrase, but Catalan commonly uses a in this type of expression.

So this is something best learned as a normal Catalan pattern rather than translated word-for-word.

Yes, you can say Jo vaig caure..., but Catalan often leaves subject pronouns out because the verb already shows the person.

• vaig caure already means I fell
• tinc already means I have

So jo is usually omitted unless you want emphasis or contrast.

For example:

• Jo vaig caure, no ell.
  I fell, not him.

In a neutral sentence, leaving it out sounds more natural.

The h in ahir is silent.

So ahir is pronounced roughly like a-IR or uh-EER, depending on accent and dialect.

The important point for a learner is:

• do not pronounce the h

This is true for Catalan h in general: it is normally silent.

In carrer, the rr is a strong rolled or trilled r.

So it is different from a single r between vowels, which is usually softer.

A rough guide:

• carrer ≈ kuh-RER / kah-RER, depending on dialect

The main learner point is:

• rr is a strong r
• the final -er sound may vary by dialect

If you are just starting, the most important thing is to make the rr stronger than an English r.

The given word order is very natural, but Catalan word order is somewhat flexible.

This version is straightforward:

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

You could move some elements for emphasis, for example:

• Vaig caure ahir al carrer...

But the original order works very well because it clearly contrasts:

• ahir = yesterday
• ara = now

That contrast is one reason the sentence sounds especially neat and natural.