Learn Arabic (Modern Standard): الولد سعيد.

In Arabic, there is no word for "is" or "are" in the present tense. To say "The boy is happy," you simply place the subject (الولد) and the adjective (سعيد) next to each other. This is called a nominal sentence, meaning a sentence that doesn't need a verb.

Why does الولد have ال (the) but سعيد does not?

This is the secret to making a nominal sentence in Arabic! The subject is usually definite (it has ال), but the adjective describing it (the predicate) must be indefinite (no ال). This difference in definiteness is what creates the invisible "is".

What would happen if I put ال on both words and wrote الولد السعيد?

If both words match and have ال, it stops being a full sentence and becomes a descriptive phrase. الولد السعيد just means "the happy boy" rather than the complete thought "The boy is happy."

Does the word سعيد change if we are talking about a girl?

Yes, adjectives in Arabic must match the gender of the noun they describe. If the subject were "the girl" (البنت), you would need to add the feminine ending (taa marbuta, ة) to the adjective, making it البنت سعيدة (The girl is happy).