Learn Italian: Il direttore ha una bella cravatta, ma non se la mette. - The manager has a beautiful tie, but doesn't put it on.

When talking about putting on clothing, Italian uses the reflexive verb mettersi (literally "to put on oneself"). Because of this, we need both the reflexive pronoun si (for "himself") and the direct object pronoun la (for "it"). When they combine before the verb, they become se la.

Why did the reflexive pronoun si change to se here?

Whenever the pronouns mi, ti, si, ci, or vi are immediately followed by a direct object pronoun like lo, la, li, or le, their ending changes from an "i" to an "e". Therefore, si + la becomes se la.

What exactly does the la refer to in this sentence?

It refers to una bella cravatta (a beautiful tie) from the first half of the sentence. Since cravatta is a feminine singular noun, we use the feminine singular pronoun la to mean "it".

Where does the negative non go when using these double pronouns?