Learn Romanian: Studenții sunt inteligenți. - The students are intelligent.

This is the difference between a plain plural and a 'the' plural! For masculine words, adding one -i makes it plural. So, student becomes studenți (students), and inteligent becomes inteligenți (intelligent). To say 'the' students, Romanian adds a second -i to the end of the noun, making it studenții. The adjective inteligenți only needs one -i because it just has to show it is plural, not definite.

How do I pronounce the single -i versus the double -ii?

A single -i at the end of a word like inteligenți is usually 'whispered' - you barely hear it as a vowel, it mostly just softens the consonant before it (turning the 't' into a 'ts' sound). But when you add the second -i for the definite article, like in studenții, that final sound becomes a full, clear vowel, pronounced like the 'ee' in the English word 'see'.

Wait, doesn't sunt mean 'I am'? Why is it translated as 'are' here?

It means 'they are' in this sentence! The Romanian verb for 'to be' is a bit special because it uses the exact same word, sunt, for both 'I am' and 'they are'. You will always know which one it is based on the subject. Since studenții means 'the students' (they), sunt has to mean 'are'.

Would the endings change if we were talking about female students?