When I open tekla ver 20 student ver suddenly closed after connect internet then open pls help offline not avalable in internet connection any time
You need to be online more or less on regular basis in order to use Tekla Structures Learning.
See this topic for more information on how Tekla Structures licensing works: https://campus.tekla.com/forum/discussion-forum/how-does-tekla-structures-learning-license-work
Are you an educator? See options for institution licensing.