Learn Romanian: Bărbații și băieții sunt aici.

This is how you say "the" for plural masculine nouns. The first -i makes the word plural (bărbat becomes bărbați for "men"). The second -i acts as the word "the" (bărbați becomes bărbații for "the men").

What are the singular forms of bărbații and băieții, and how did their spelling change?

The singular forms are bărbat (man) and băiat (boy). To make them plural, you add an -i, but Romanian also changes some vowels inside the words to make them easier to say. So bărbat becomes bărbați, and băiat becomes băieți. Then we add the second -i for "the".

How do I pronounce the double i at the end of bărbații and băieții?

A single -i at the end of a plural word like bărbați is usually whispered or "short"—it just slightly changes the sound of the consonant before it. But when you add the second -i to make bărbații, it becomes a full, clear "ee" sound, like the "ee" in the English word "see".

Wait, doesn't sunt mean "I am"? Why is it used here for "the men and the boys"?