Learn Italian: Mi lavo la testa la mattina. - I wash my head in the morning.

This is because the sentence uses a reflexive verb. In Italian, everyday routines like washing, waking up, or getting dressed are actions you do to yourself. Mi means "myself", so mi lavo literally means "I wash myself".

Where did the -si from the dictionary verb lavarsi go?

When you conjugate a reflexive verb in the present tense, the -si ending detaches, changes to match the person doing the action (mi for "I"), and moves in front of the verb. Then, you just conjugate the base verb (lavare) normally to get lavo.

Why does it say la testa (the head) instead of using a word for "my"?

In Italian, you don't usually use possessive words (like "my" or "your") with body parts or clothing. Because you already used the reflexive pronoun mi to show the action is happening to yourself, it is already obvious whose head is being washed! You just use the definite article (la) instead.

Why does la mattina translate to "in the morning" without a word for "in"?

To say "in the morning", "in the afternoon", or "in the evening", Italian often just uses the definite article with the time of day: la mattina, il pomeriggio, la sera. You do not need to add a preposition to translate "in" here.