Learn Italian: Mi lavo i capelli con lo shampoo.

In Italian, when you do an action to your own body parts, you use a reflexive verb. Mi lavo literally means I wash myself. Instead of saying I wash my hair with a possessive word, Italians say I wash myself the hair.

Why is capelli plural when the English translation uses the singular word hair?

In English, hair on your head is a singular, uncountable concept. In Italian, capelli is plural and countable. You are talking about all the individual hairs on your head! If you said il capello, it would mean just one single strand of hair.

Why isn't there a word for my before capelli?

Because the reflexive word mi already shows whose hair it is. Adding a possessive word like miei would be repetitive. When talking about body parts, Italians almost always drop the possessive and just use the definite article, which is i (the) in this sentence.

Why is the article lo used before shampoo instead of il?

The article lo is used for masculine singular words that start with an s followed by another consonant. The sh sound in shampoo follows this rule. Also, Italian often requires definite articles before general substances or materials, whereas English just says with shampoo.