Mi prima cumplirá diecinueve en agosto y quiere celebrarlo en la playa.

In Spanish, primo is male cousin and prima is female cousin. So mi prima means my (female) cousin.

Spanish often marks gender on family words:

• primo / prima = cousin
• tío / tía = uncle / aunt
• hermano / hermana = brother / sister

Cumplirá is the future tense of cumplir. In this sentence, cumplir diecinueve means to turn nineteen.

So:

• cumple diecinueve = she turns / is turning nineteen
• cumplirá diecinueve = she will turn nineteen

Even though cumplir can mean to fulfil or to complete in other contexts, with age it means to turn a certain age.

It comes from the infinitive cumplir + a future ending.

Future tense endings in Spanish are added directly to the infinitive:

• cumpliré = I will turn / fulfil
• cumplirás = you will turn / fulfil
• cumplirá = he/she will turn / fulfil
• cumpliremos = we will turn / fulfil
• cumpliréis = you all will turn / fulfil
• cumplirán = they will turn / fulfil

The accent in cumplirá shows the stress falls on the last syllable.

Because in Spanish, after cumplir with an age, años is often left out when it is obvious.

So both are possible:

• cumplirá diecinueve
• cumplirá diecinueve años

Both mean the same thing. Omitting años sounds completely natural.

Spanish uses en with months to mean in:

• en agosto = in August
• en enero = in January

So:

• en agosto = in August

This is the normal preposition for months, seasons, and many time expressions.

Because the sentence is talking about two slightly different things:

• cumplirá diecinueve en agosto = a future event
• quiere celebrarlo = her current wish or intention

So the meaning is:

• she will turn nineteen in August
• and she wants to celebrate it on the beach

Spanish often mixes tenses like this when a present desire relates to a future event.

You could also hear:

• Mi prima cumplirá diecinueve en agosto y querrá celebrarlo en la playa

But that sounds more like a prediction: she will want to celebrate it.

Celebrarlo = celebrar + lo

• celebrar = to celebrate
• lo = it

When an object pronoun is attached to an infinitive, it becomes one word:

• celebrarlo = to celebrate it
• hacerlo = to do it
• verla = to see her

So quiere celebrarlo literally means she wants to celebrate it.

Because lo refers to the event or fact of turning nineteen / the birthday occasion, not to mi prima.

If lo referred to prima, it would have to be la, because prima is feminine. But that is not what is happening here.

So celebrarlo means:

• celebrate it = celebrate the birthday / turning nineteen

You can think of lo here as referring to an understood masculine idea like:

• el cumpleaños or more generally
• eso / that

It refers to the occasion just mentioned: her turning nineteen, or implicitly her birthday.

Spanish often avoids repeating the noun if it is already clear from context. So instead of saying:

• quiere celebrar su cumpleaños en la playa

the sentence says:

• quiere celebrarlo en la playa

Both are natural.

Because en la playa expresses the location where the celebration will happen: on/at the beach.

• en la playa = at/on the beach
• a la playa = to the beach

So:

• quiere celebrarlo en la playa = she wants to celebrate it at the beach
• quiere ir a la playa = she wants to go to the beach

A simple way to remember it:

• a often shows movement toward a place
• en often shows being or happening in a place

Yes. Both are natural.

• cumplirá diecinueve = she will turn nineteen
• va a cumplir diecinueve = she is going to turn nineteen

The second form is the near future and is very common in everyday speech. The simple future (cumplirá) is also perfectly normal, especially in writing or more neutral narration.

Mostly yes, but Spanish can be a little more flexible about leaving out what is being celebrated if the context is clear.

Examples:

• celebrar un cumpleaños = to celebrate a birthday
• celebrar una boda = to celebrate a wedding
• celebrarlo = to celebrate it

In this sentence, Spanish does not repeat birthday because the previous clause already gives the idea.

Yes. For example:

• Mi prima cumplirá diecinueve en agosto y quiere celebrar su cumpleaños en la playa.

That is also correct. It is just a bit more explicit.

Using celebrarlo sounds natural because Spanish often prefers a pronoun when the meaning is already clear.

Yes. Diecinueve is written as one word.

Some useful nearby numbers:

• dieciséis = 16
• diecisiete = 17
• dieciocho = 18
• diecinueve = 19

Notice that some of these have written accents, such as dieciséis, but diecinueve does not.

No. Cumplir has several meanings, depending on context. Common ones include:

• cumplir años = to have a birthday / turn a certain age
• cumplir una promesa = to keep a promise
• cumplir una obligación = to fulfil an obligation
• cumplir treinta años = to turn thirty

So in this sentence, context tells you it is the age meaning.