Atrapé la pelota ayer.

Because it is the first-person singular preterite form of atrapar: yo atrapé.

The accent serves two purposes:

• It shows the stress goes on the last syllable: a-tra-PÉ
• It distinguishes this form from atrape, which could be a different form, such as a subjunctive or command-related form depending on context

In regular -ar verbs, the yo form of the preterite ends in -é:

• hablé
• miré
• atrapé

Atrapé is the preterite tense.

In Spanish, the preterite is used for a completed action in the past, especially when it happened at a specific time or is seen as a finished event.

That fits well with ayer, because yesterday is a clearly finished time period.

So Atrapé la pelota ayer uses the normal tense for:

• one completed action
• at a definite time in the past

Because Spanish often omits subject pronouns when the verb ending already makes the subject clear.

In atrapé, the ending -é already tells you the subject is yo.

So:

• Atrapé la pelota ayer
• Yo atrapé la pelota ayer

Both are correct, but the version without yo is often more natural unless you want emphasis or contrast.

For example, you might include yo if you want to stress I:

• Yo atrapé la pelota, no él.

Spanish uses the definite article much more often than English does.

So where English might say:

• I caught the ball or even in some contexts
• I caught ball would be impossible

Spanish normally says:

• atrapé la pelota

The article la is used because pelota is a feminine singular noun.

You could sometimes hear other articles depending on meaning:

• una pelota = a ball
• la pelota = the ball

In this sentence, la pelota is the normal way to refer to the object that was caught.

Because in this sentence la is an article, not a pronoun.

Here it comes directly before the noun pelota:

• la pelota

That structure means the ball.

If la were a direct object pronoun meaning her or it, it would usually appear before the verb, not directly attached to a noun:

• La atrapé ayer. = I caught her/it yesterday.

So in Atrapé la pelota ayer, la is clearly the article.

Yes, Ayer atrapé la pelota is also correct.

Spanish word order is more flexible than English word order. The most neutral order here is:

• Atrapé la pelota ayer

But you can move ayer for emphasis or style:

• Ayer atrapé la pelota
• La pelota la atrapé ayer (more marked, with emphasis or contrast)

In everyday Spanish, both Atrapé la pelota ayer and Ayer atrapé la pelota sound natural.

A Spain Spanish approximation would be:

• a-tra-PE la pe-LO-ta a-YER

A few pronunciation points:

• tr in atrapé is pronounced together, with a tapped r
• the stress in atrapé is on the last syllable because of the accent
• ll does not appear here, but y in ayer is usually pronounced like a consonant sound at the start of the syllable
• in much of Spain, y in ayer sounds somewhat like the y in yes, though exact pronunciation varies

If you want to sound natural, focus especially on:

• a-tra-PE
• a-YER

The infinitive is atrapar.

This is a regular -ar verb. Its preterite endings are the normal ones for -ar verbs:

• yo atrapé
• tú atrapaste
• él/ella atrapó
• nosotros atrapamos
• vosotros atrapasteis
• ellos atraparon

So once you know atrapar, atrapé follows a predictable pattern.

Yes, in Spain you often could use coger in a sentence like this, depending on the exact meaning.

• atrapar = to catch, to grab after pursuit or movement, often focusing on successfully catching something
• coger = to take, grab, catch

In Spain, coger la pelota can be perfectly normal. However:

• atrapar la pelota strongly suggests catching the ball
• coger la pelota can sound more like taking/picking up/getting hold of the ball, depending on context

Also, an important cultural note:

• In Spain, coger is common and neutral
• In many parts of Latin America, coger can be vulgar, so learners are often warned about it

Since you specified Spanish from Spain, coger is fine there, but atrapar is a very clear choice for catch.

This is the difference between preterite and imperfect.

• atrapé = I caught / I did catch
  A completed action
• atrapaba = I was catching / I used to catch / I caught repeatedly
  An ongoing, habitual, or background action

So:

• Atrapé la pelota ayer = one completed event yesterday
• Atrapaba la pelota todos los días = I used to catch the ball every day

Because ayer points to a specific finished time, atrapé is the natural choice here.

Yes, and often the tense would still stay in the preterite if the action is seen as completed.

Examples:

• Atrapé la pelota esta mañana.
• Atrapé la pelota el lunes.
• Atrapé la pelota hace dos horas.

All of those describe a finished action.

But if the time expression suggests habit or repeated action, Spanish may prefer the imperfect:

• Cuando era niño, atrapaba la pelota muy bien.

So the tense depends not only on the time word, but on whether the action is viewed as:

• completed once → usually preterite
• habitual/ongoing/background → often imperfect

Yes. Pelota is a feminine noun, so it takes feminine articles and adjectives:

• la pelota
• una pelota
• la pelota roja

That is why the sentence uses la, not el.

This grammatical gender is a property of the noun itself. It does not mean the ball is biologically female; it is just how the noun behaves grammatically in Spanish.