Learn Italian: Ci sono molti fiori nei vasi. - There are many flowers in the vases.

Nei means "in the". It is an articulated preposition, which means it combines the word in with a definite article ("the"). Here, it combines in + i to make nei. We use the article i because vasi (vases) is a plural masculine noun starting with a normal consonant.

Why does the sentence start with ci sono instead of just sono?

Ci sono translates to "there are". If you only used sono, the sentence would mean "They are many flowers in the vases." Whenever you want to say that multiple things exist somewhere, you use ci sono. For a single thing ("there is"), you would use c'è.

How do you know fiori is plural? What is the singular word for flower?

The singular word is fiore (flower). In Italian, nouns that end in -e in the singular change their ending to -i in the plural, whether they are masculine or feminine. Since fiore is a masculine noun, it becomes fiori in the plural.

Why is it molti and not molto?