We use the when we talk about something and the speaker and the listener know which one we are talking about.
Would you like the cheese or the chicken sandwich?
Could I have the cheese sandwich, please?
Did you bring the laptop? (=The speaker and the listener both know which laptop they are talking about.)
We use the when we talk about something for the second time.
She's got a bicycle. The bicycle is blue.
We also use the before ordinal numbers in dates, especially when we are speaking.
the third ofJanuary
the tenth of May
We use the when we talk about musical instruments.
I play the piano.
She plays the guitar.