Cantamos feliz cumpleaños al abuelo.

Cantamos is the nosotros form of cantar in the present tense: we sing.

So:

• yo canto = I sing
• tú cantas = you sing
• él/ella canta = he/she sings
• nosotros cantamos = we sing

Be aware that cantamos can also mean we sang in the simple past (preterite). Usually context tells you which meaning is intended.

In Spanish, the simple present can often cover both ideas depending on context.

So cantamos feliz cumpleaños al abuelo can mean:

• we sing Happy Birthday to Grandpa
• we are singing Happy Birthday to Grandpa

If you want to emphasize right now, Spanish can also use:

• estamos cantando feliz cumpleaños al abuelo = we are singing Happy Birthday to Grandpa

But the simple present is very common and natural.

Because cantarle algo a alguien usually marks the person receiving the song with a.

Here, el abuelo becomes a el abuelo, which contracts to al abuelo.

So:

• Cantamos feliz cumpleaños al abuelo = We sing Happy Birthday to Grandpa

This a is not exactly the same as English to, but in this sentence it works very similarly.

In Spanish, a + el always contracts to al.

So:

• a el abuelo → al abuelo

This is a standard contraction, just like:

• de + el → del

But this only happens with the article el, not with names or other words:

• a la abuela
• a los niños
• a María

Feliz cumpleaños is the standard fixed expression for Happy Birthday in Spanish.

Literally, word order in Spanish often differs from English. Even though feliz means happy and cumpleaños means birthday, the expression is learned as a set phrase:

• ¡Feliz cumpleaños! = Happy Birthday!

You may also hear the song line Cumpleaños feliz, especially in the birthday song itself, but in everyday поздравление-style use, feliz cumpleaños is the normal phrase.

Because feliz cumpleaños is being used as the name/content of what is sung, not as a regular noun phrase with an article.

Spanish often leaves out the article in expressions like this:

• Cantar feliz cumpleaños
• Decir gracias
• Desear buena suerte

It works a bit like naming the words or formula being expressed.

Yes. In this sentence, feliz cumpleaños is what is being sung, so it functions as the direct object.

A simple breakdown:

• Cantamos = verb
• feliz cumpleaños = what we sing
• al abuelo = to whom we sing it

So the structure is basically:

We sing + Happy Birthday + to Grandpa

Yes, absolutely. That is very natural.

• Le cantamos feliz cumpleaños al abuelo

Here, le is the indirect object pronoun meaning to him. Spanish often uses the pronoun and the full noun together:

• Le cantamos feliz cumpleaños al abuelo
• literally: We sing Happy Birthday to him, to Grandpa

This is extremely common in Spanish and often sounds more complete or natural.

Usually, al abuelo is better here.

Why? Because abuelo is the recipient of the action, and Spanish normally marks that with a:

• Cantamos feliz cumpleaños al abuelo

Using para el abuelo would sound more like for Grandpa in the sense of intended for him, not the usual way to express we sing to Grandpa.

So for this sentence, al abuelo is the normal choice.

It can mean both, depending on context and tone.

• abuelo = grandfather / grandpa
• el abuelo = the grandfather / grandpa

In many everyday situations, abuelo often feels like grandpa in English, but the exact translation depends on the family tone and context.

Spanish often uses the definite article with family nouns when referring to a specific person in the family.

So:

• el abuelo = Grandpa / the grandfather

In English, we often drop the article when using family titles as names, but Spanish may keep it in many structures, especially after prepositions:

• al abuelo
• con la abuela
• para el tío

That is normal Spanish usage.

Normally, no. In Spanish, ordinary words and phrases are not capitalized the way English often capitalizes titles or special expressions.

So the normal spelling is:

• Cantamos feliz cumpleaños al abuelo.

You would only capitalize if it began the sentence or was part of a title.

Cumpleaños already looks plural, but as a word it is used the same in singular and plural forms.

Examples:

• Mi cumpleaños es hoy. = My birthday is today.
• Tus cumpleaños siempre son divertidos. = Your birthdays are always fun.

In feliz cumpleaños, you do not change it. That whole phrase is fixed.

Yes, Spanish allows some flexibility, though the original order is very natural.

You might also hear:

• Le cantamos feliz cumpleaños al abuelo.
• Al abuelo le cantamos feliz cumpleaños.

The basic meaning stays the same, but changing the order can shift emphasis.

The most neutral version is:

• Cantamos feliz cumpleaños al abuelo.

A very common everyday version would be:

• Le cantamos el cumpleaños al abuelo
• Le cantamos feliz cumpleaños al abuelo
• Le cantamos Las Mañanitas al abuelo (in places where that song is traditional)

Your original sentence is correct and clear. Adding le often makes it sound more naturally Spanish in conversation:

• Le cantamos feliz cumpleaños al abuelo.