the is used with musical instruments

Can you play the piano?

The guitar is my favorite instrument.

The computer is the most important invention.

The telephone was invented by Alexander Graham Bell.

but

She bought a guitar./ a computer / a telephone.

The + adjective gives a plural meaning.

A network of volunteers is working to distribute food and blankets to the homeless.

The dead can’t talk.

The old are more experinced than the young.

the is used with the word same

Their interests, and his own are the same.

Some people always choose the same numbers in the lottery.

You’re just repeating the same points over and over.