Las semillas empezaron a germinar.

Las is the plural feminine definite article, meaning the.

It is used because semillas (seeds) is:

• plural → more than one seed
• feminine → the noun semilla is feminine

So:

• la semilla = the seed
• las semillas = the seeds

In Spanish, nouns have grammatical gender. Semilla is a feminine noun, so it takes:

• la in the singular
• las in the plural

A lot of nouns ending in -a are feminine, and semilla follows that common pattern.

Examples:

• la semilla
• una semilla
• las semillas

Empezaron is the preterite tense of empezar.

Here it means they started or they began.

The verb form is:

• empezar = to begin / to start
• empezaron = they started / they began

Because the subject is las semillas (the seeds), the verb is in 3rd person plural.

This is about the difference between the preterite and the imperfect.

• empezaron = they started / they began
  This presents the beginning as a completed event.
• empezaban = they were starting / used to start
  This sounds more ongoing, descriptive, or habitual.

In Las semillas empezaron a germinar, the speaker is talking about the moment or event when the seeds began to germinate, so preterite is the natural choice.

Because empezar is normally followed by a + infinitive when it means to start doing something.

So the pattern is:

• empezar a + infinitive
• comenzar a + infinitive

Examples:

• Empezó a llover = It started to rain
• Empezaron a hablar = They started to speak
• Las semillas empezaron a germinar = The seeds started to germinate

You generally do not say empezaron germinar.

Germinar is an infinitive, meaning to germinate.

After empezar a, Spanish uses the infinitive:

• empezaron a germinar = started to germinate

So the structure is:

• main verb: empezaron
• preposition: a
• infinitive: germinar

Yes. Germinar is a regular -ar verb.

That means it follows the normal -ar patterns.

For example:

• germino = I germinate
• germinó = it germinated
• germinaron = they germinated

In your sentence, though, it stays in the infinitive because it comes after a:

• empezaron a germinar

Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action.

Here, empezaron tells you the subject is they (or you all, depending on context), and las semillas is already stated explicitly.

So Spanish does not need ellas here.

• Las semillas empezaron a germinar = The seeds started to germinate

Adding ellas would usually be unnecessary:

• Las semillas, ellas empezaron a germinar → unnatural in most normal contexts

Yes. Comenzaron a germinar is a very natural alternative.

• empezaron a germinar
• comenzaron a germinar

Both mean started to germinate or began to germinate.

Empezar and comenzar are very similar here. Empezar is often a bit more common in everyday speech, but both are correct.

Yes, but the meaning changes slightly.

• Las semillas empezaron a germinar = The seeds started to germinate
• Las semillas germinaron = The seeds germinated

The first focuses on the beginning of the process.
The second says the action happened, without highlighting the start.

So they are related, but not identical.

This is actually the normal spelling.

The infinitive is empezar, with z.
In the preterite plural form, it stays:

• empezaron

No spelling change is needed here.

A useful thing to know is that empezar is a stem-changing verb in some tenses:

• empiezo
• empiezas
• empieza

But in the preterite, that stem change does not happen:

• empecé
• empezó
• empezaron

So empezaron is exactly what you should expect.

A Spain pronunciation would be approximately:

las seh-MEE-yas em-peh-THA-ron a kher-mee-NAR

A few key points:

• semillas:
  
  • ll is usually pronounced like y in modern Spanish from Spain
• empezaron:
  
  • the z in Spain is usually pronounced like th in think
• germinar:
  
  • the g before e sounds like a strong throaty sound, similar to the ch in Scottish loch or German Bach

A more detailed breakdown:

• Las = las
• semillas = seh-MEE-yas
• empezaron = em-peh-THA-ron
• a = a
• germinar = kher-mee-NAR

Yes, it is completely natural.

Las semillas empezaron a germinar is a normal, clear way to say that the seeds began the germination process.

It sounds like standard Spanish and would be understood easily in Spain and elsewhere in the Spanish-speaking world.