Learn Italian: Gli uomini si lavano le mani lentamente.

Uomo is one of the most common irregular nouns in Italian. Instead of just changing the final -o to -i, it completely changes its root by adding a syllable in the plural: uomini. Notice that it uses the plural article gli because it starts with a vowel.

Mani ends in -i, but the article is le. Is it feminine?

Yes! La mano (the hand) is a famous exception in Italian. It ends in -o but is feminine. In the plural, the ending changes to -i like most plurals do, but because it is feminine, it uses the feminine plural article le, becoming le mani.

Why does it say si lavano le mani (wash themselves the hands) instead of using a word for "their" hands?

In Italian, when you do an action to your own body parts or clothing, you don't usually use possessive words like "my" or "their". Instead, you use a reflexive verb (here, si lavano) and the definite article (le mani). It literally translates to "they wash the hands to themselves."

How does the word lentamente (slowly) get its ending?