Learn Romanian: Orașele sunt mari.

It comes from the neuter noun oraș (city). To make it plural, we add the suffix -e, giving us orașe (cities). To say 'the cities', we attach the definite article -le to the end, resulting in orașele.

Isn't -le the definite article for feminine plural nouns? Why is it used here?

Neuter nouns in Romanian have a special rule: they act like masculine nouns in the singular, but like feminine nouns in the plural. Since orașe is a plural neuter noun, it behaves like a feminine word and takes the feminine plural definite article -le.

Why is the word for big mari instead of mare?

Adjectives must match the noun they describe. Because orașele is plural, the adjective must also be plural. For Romanian adjectives that end in -e in the singular, like mare, the plural form changes the ending to -i, making it mari.

Doesn't sunt mean 'I am'? Why is it used for 'the cities'?

The Romanian verb a fi (to be) uses the exact same form for both 'I am' (eu sunt) and 'they are' (ei/ele sunt). Since 'the cities' is a 'they', we use sunt here to mean 'are'.