Después de la caída, tenía mucha hinchazón en el tobillo y pensé que era un esguince.

Because después normally needs the preposition de before a noun.

• después de la caída = after the fall
• después by itself usually means afterwards / later

So:

• Después de la caída, ... = After the fall, ...
• Después, ... = Afterwards, ...

That de is required here.

In Spanish, articles are used more often than in English. La caída means the fall.

Here, the speaker is referring to a specific fall that both speaker and listener can identify from context. That is why la sounds natural.

Without the article, caída would sound incomplete in this sentence.

Caída is a noun meaning fall. It comes from the verb caer, which means to fall.

Examples:

• caer = to fall
• me caí = I fell
• la caída = the fall

So the sentence starts with a noun phrase, después de la caída = after the fall, rather than a full clause like después de que me caí = after I fell.

Tenía is the imperfect tense of tener, while tuve is the preterite.

Spanish often uses the imperfect to describe a condition or state in the background, especially when talking about how something was at a certain moment.

Here, tenía mucha hinchazón en el tobillo describes the speaker’s condition after the fall:

• tenía = I had / I was experiencing

It presents the swelling as an ongoing state, not as a single completed event.

If you said tuve mucha hinchazón, it would sound more like I had a lot of swelling as a completed fact, which is possible in some contexts, but tenía is more natural in this kind of description.

Because pensé refers to a specific moment when the speaker came to that conclusion: I thought.

• pensé = preterite, a completed thought or conclusion
• pensaba = imperfect, more like I was thinking or I used to think

In this sentence, the speaker is talking about one particular reaction after the fall:

• ... y pensé que... = ... and I thought that...

So the preterite fits well.

This is a very common question.

After verbs like pensé, Spanish often uses the imperfect when describing what something seemed to be or what someone believed it was at the time.

• pensé que era un esguince = I thought it was a sprain

Here, era describes the speaker’s interpretation of the injury.

Using fue would sound much less natural in this context because ser in the preterite tends to present something as a completed, bounded event, and that does not fit well with identifying an injury.

So era is the normal choice.

Because mucho/a is used with nouns, while muy is used with adjectives or adverbs.

• mucha hinchazón = a lot of swelling
• muy hinchado = very swollen

So:

• tenía mucha hinchazón = correct
• tenía muy hinchazón = incorrect

Compare:

• mucho dolor = a lot of pain
• muy doloroso = very painful

Because hinchazón here is being treated as an uncountable noun, like swelling in English.

So mucha hinchazón means a lot of swelling, not many swellings.

Spanish often uses singular abstract or medical nouns this way:

• mucho dolor = a lot of pain
• mucha inflamación = a lot of inflammation
• mucha hinchazón = a lot of swelling

Because en is the natural preposition here for location: in/on the ankle area.

• hinchazón en el tobillo = swelling in the ankle / swelling around the ankle

Spanish often uses en to indicate where something is located in the body.

Spanish often uses the definite article with body parts when it is already clear whose body part is meant.

So instead of saying my ankle, Spanish often says the ankle:

• me duele el tobillo = my ankle hurts
• tenía hinchazón en el tobillo = I had swelling in my ankle

Because the subject is the speaker, it is obvious that it is the speaker’s ankle. Using mi tobillo is possible, but el tobillo is more natural in many cases.

Esguince means sprain.

It is a masculine noun:

• un esguince = a sprain

A very common full expression is:

• un esguince de tobillo = an ankle sprain

In the sentence, de tobillo is omitted because en el tobillo has already told us where the problem is.

Yes, that would also be very natural.

There is a slight difference in focus:

• pensé que era un esguince = I thought it was a sprain
  
  • focus on identifying the injury
• pensé que tenía un esguince = I thought I had a sprain
  
  • focus on the speaker having that injury

Both are correct and common.

Yes, absolutely. That is a very natural alternative.

• tenía mucha hinchazón en el tobillo = I had a lot of swelling in my ankle
• tenía el tobillo muy hinchado = my ankle was very swollen / I had a very swollen ankle

The first uses the noun hinchazón.
The second uses the adjective hinchado.

Both are good Spanish.

Because Spanish often combines these tenses for different functions:

• tenía = imperfect, background condition
• pensé = preterite, a specific reaction or event
• era = imperfect, how the injury was understood or identified at that moment

So the structure is very typical:

1. background state: tenía mucha hinchazón
2. completed thought/reaction: pensé
3. description/classification: era un esguince

This mix of tenses is one of the most important patterns in Spanish narration.

The original sentence is perfectly good Spanish, but in conversation you might also hear something like:

• Después de la caída, tenía el tobillo muy hinchado y pensé que era un esguince.
• Después de caerme, se me hinchó mucho el tobillo y pensé que era un esguince.

These versions are a bit more colloquial, especially después de caerme and se me hinchó mucho el tobillo.

The original sentence is still completely natural and clear.