Student or Learner
I would like to get a brief explanation of double-verb. and used to+ infinitive and used to+ gerund in semantic point of view.
Welcome to the forum, zaitun.
I'm afraid I don't know what you mean by 'double verb'. Where did you find this term?
If you want to know the meaning if 'used to' and 'BE used to', start here: http://onelook.com/browse.shtml. Come back to this thread if you have any problems with the definitions you find.