Learn Romanian: Lacurile și râurile sunt mari. - The lakes and the rivers are big.

In Romanian, the word for 'the' (the definite article) attaches to the end of the noun instead of coming before it. For plural neuter and feminine nouns, this ending is usually -le. So, lacuri (lakes) becomes lacurile (the lakes), and râuri (rivers) becomes râurile (the rivers).

What gender are the base words lac and râu?

They are both neuter nouns. In Romanian, neuter nouns act like masculine nouns when there is just one (singular) and like feminine nouns when there are two or more (plural). You can see the common neuter plural ending -uri in both lacuri and râuri.

Doesn't sunt mean 'I am'? Why is it used here for 'the lakes and rivers are'?

It means both! The verb a fi (to be) is special in Romanian. The exact same form, sunt, is used for both 'I am' (eu sunt) and 'they are' (ei/ele sunt). Here, it translates to 'are' because the subject is plural ('they').

Why is the word for big mari instead of mare?

Adjectives in Romanian have to match the noun in number (singular or plural). Because the lakes and rivers are plural, the singular adjective mare (big) changes to its plural form, mari.