Usage: need
Use need when it is necessary to do it, you need to do it.
For examples:
* "Need to" is something like "must". It's not quite the same, but it has a
very similar meaning.
They need to argue with him or he won't do what they want.
= They must argue with him, or he won't do what they want.
"Need" means something like "must have".
You need eggs to bake this cake.
= You must have eggs to bake this cake.
Wrap-up
could do something -> It's possible for someone to do something.
must do something -> It's absolutely necessary for someone to do
something.
need to do something -> It's important to do something.
has to/ have to do something -> It's absolutely necessary for
someone to do something and someone else requires it.
mustn't do something -> It's forbidden for someone to do something.