Modal verbs

References:

1. 
   Leech G., Cruickshank B., Ivanic R.: An A-Z of English Grammar & Usage. Harlow: Longman, 2004.
   
2. 
   Murphy R.: English Grammar in Use. Cambridge: Cambridge University Press, 2006.
   
3. 
   A Linguistic Study of the English Verb, Longmans, 1965.
   
4. 
   Comrie, Bernard, Tense, Cambridge Univ. Press, 1985, pp. 21, 47-48."UltraLingua Online Dictionary & Grammar.
   
5. 
   David Rubin, "might could (double modal)", The Mavens' Word of the Day, Random House, November 20, 2000.
   
6. 
   Fleischman, Suzanne, The Future in Thought and Action, Cambridge Univ. Press, 1982, pp. 86-97.
   
7. 
   Fowler's Dictionary of Modern English Usage (4th ed.). Oxford University Press. p. 853. ISBN 978-0- 199-66135-0.
   
8. 
   Garner, Bryan A. (2003). Garner's Modern American Usage. Oxford University Press. p. 810. ISBN 978-0-19-516191-5. Butterfield, eds. (2015).
   
9. 
   Kenneth G. Wilson, "Double Modal Auxiliaries", The Columbia Guide to Standard American English, 1993.