apart, part, a part

Apart or apart ( one word ) means separated, in space or in time--
They write to each other every day when they're apart.
The two sisters were born a year apart.

Apart also means in pieces, literally or figuratively--
He took the clock apart, and now he can't put it back together.
"You're tearing me apart!"

Part or part means a section or piece, literally or figuratively--
He finished part of the job, but then he quit.
He ate part of his lunch, but left the rest.
Part of this is his fault!

Part or part also means to separate or divide--
I have something to say before we part.
He asked the barber to part his hair in the middle.

Part may also mean a role in a play or film, or a role in a real life--
This time he has a speaking part.
This part will be easy to learn.

A part or a part ( two words ) means only one ( a ) section or piece, literally or figuratively--
We need a part to fix the car.
He felt like he had left a part of himself behind, in his old home town.
"You'll always be a part of me."

A part may mean a role on a film or play--
He got a part in the new play.
He hopes to get a part in a movie.

Now that you know that, you can say--
"They don't like to be apart, but loneliness is only a part of their problem--the other part is immaturity."