taut, tight
He pulled the string tight.

loose
tangled
He had hair like tangled yellow string.

bit (esp. BrE), length, piece
I cut a length of string to tie up the package.

ball

tie, tie sth up with
Tie the string around the package.

undo, untie
wind
He wound the string into a ball.

dangle (sth) on
Next to the phone, there was a pencil dangling on a string.

vest (BrE)

in the string
There's a knot in the string.

string of
a string of pearls

open
guitar, violin, etc.
A, G, etc.

tune
loosen, tighten
pluck
change, replace
break

break
vibrate

instrument

on a/the string
Play it on the G string.