If you have been drinking enough fluids but cannot urinate easily even with a full bladder, it could be an issue with the nerves, or it could be an infection. There are some STIs (treatable, happily)...
Able means "can do". Unable means "cannot do". This could refer to an action, behavior, speech, etc. Examples: I'm unable to say why the experiment failed.I'm unable to walk.I'm unable to hear.I'm...