Learn Arabic (Modern Standard): البنت حزينة.

In Arabic, there is no word for "is" or "are" in the present tense. You simply put the subject (البنت, the girl) and the describing word (حزينة, sad) next to each other to make a full sentence.

How do we know this means "The girl is sad" and not "The sad girl"?

This is a very important rule for adjectives! To make a full sentence like "The girl is sad", the noun has "the" attached (البنت) but the adjective does not (حزينة). If you wanted to say the phrase "the sad girl", both words would need "the" to match: البنت الحزينة.

Why does the adjective حزينة have a ة (taa marbuuTa) at the end?

Adjectives must match the gender of the noun they describe. Because البنت (the girl) is feminine, we add the feminine ending ة to the basic masculine adjective حزين (sad) to make it حزينة.

Can I change the word order and say حزينة البنت?

In a basic sentence like this one, the order matters. The subject you are talking about (البنت) must come first, followed by the adjective (حزينة) that gives us information about it.