El gato empieza a ronronear cuando mi abuela lo acaricia.

Because empieza a ronronear means starts to purr or begins purring, while ronronea simply means purrs / is purring.

So:

• El gato ronronea = The cat purrs / is purring.
• El gato empieza a ronronear = The cat starts to purr.

The sentence is focusing on the beginning of the action.

Because empezar normally uses the pattern empezar a + infinitive.

So:

• empieza a ronronear
• empieza a hablar
• empieza a correr

That a is just part of the construction. In English we often use to after start as well: starts to purr.

Because empezar is a stem-changing verb in the present tense. The e in the stem changes to ie in most forms.

Present tense of empezar:

• yo empiezo
• tú empiezas
• él / ella / usted empieza
• nosotros empezamos
• vosotros empezáis
• ellos / ellas / ustedes empiezan

So here, with el gato, we need the él/ella form: empieza.

Because the sentence describes a habitual or general situation:

Whenever my grandmother pets him, the cat starts purring.

This is not talking about one specific event only. It is saying what usually happens.

Spanish often uses the present tense for this kind of repeated action, just like English can use the present for general truths or habits.

Because cuando can mean when or whenever, and in this sentence it introduces a repeated action.

• cuando mi abuela lo acaricia = when / whenever my grandmother pets him

For habitual actions, Spanish uses the present after cuando.

A useful comparison:

• Cuando mi abuela lo acaricia, el gato empieza a ronronear.
  Habitual: whenever she pets him, he starts purring.

• Cuando mi abuela lo acaricie, el gato empezará a ronronear.
  Future idea: when she pets him in the future, he will start purring.

Lo is the direct object pronoun meaning him or it, referring to el gato.

So:

• mi abuela acaricia al gato = my grandmother pets the cat
• mi abuela lo acaricia = my grandmother pets him / it

Spanish often replaces a repeated noun with a pronoun, just as English does.

Because with a conjugated verb, object pronouns usually go before the verb.

So:

• lo acaricia
• la ve
• los llama

You cannot normally put it after a finite verb like acaricia.

Compare this with infinitives and gerunds, where pronouns can attach to the end:

• quiere acariciarlo
• está acariciándolo

But in your sentence, acaricia is a fully conjugated verb, so lo goes before it.

In some parts of Spain, yes, you may hear le for a male person or male animal because of leísmo. However, the standard form taught to learners is:

• lo acaricia

That is the safest form to use, especially in writing and in most teaching materials.

So:

• standard / widely taught: lo acaricia
• heard in some parts of Spain: le acaricia

Because they are working differently in the sentence.

• el gato = a specific cat, so it takes the definite article
• mi abuela = my grandmother, and possessives like mi, tu, su normally replace the article

So Spanish usually says:

• mi madre
• mi hermano
• mi abuela

not la mi abuela.

Because Spanish often leaves subject pronouns out when the meaning is already clear from the verb form or context.

So instead of saying:

• El gato empieza...
• mi abuela lo acaricia...

Spanish does not need:

• Él empieza...
• Ella lo acaricia...

In fact, adding subject pronouns unnecessarily can sound unnatural unless you want emphasis or contrast.

Acariciar means to stroke, to caress, or, with animals, very often to pet.

In this sentence, pet is the most natural English choice:

• mi abuela lo acaricia = my grandmother pets him

With people, acariciar can sound more like stroke or caress, depending on context.

A few pronunciation points matter here:

• rr in ronronear is a strong rolled or trilled r
• the word is roughly ron-ro-ne-AR
• the stress falls on the last syllable -ar

In Spain, a rough guide would be:

ron-roh-neh-AR

Also:

• gato has a normal hard g
• acaricia in Spain is pronounced with the c before i as a th sound: roughly aka-REE-thya
• in much of Latin America, that same c would sound like s

Yes. That is completely natural and means the same thing.

Spanish allows both orders:

• El gato empieza a ronronear cuando mi abuela lo acaricia.
• Cuando mi abuela lo acaricia, el gato empieza a ronronear.

If the cuando clause comes first, you normally write a comma after it. The second version may sound slightly more focused on the condition or trigger: when my grandmother pets him...