Must, Have to, Need to, Don’t Have to, Needn’t modal verbs Should, Shouldn’t, Ought To modal verbs

References

1. 
   Palmer, F. R., Mood and Modality, Cambridge University Presents, 2001, p. 33
   
2. 
   A Short Overview of English Syntax (Rodney Huddleston), section 6.5d
   
3. 
   Palmer, op. cit., p. 70. The subsequent text shows that the intended definitions were transposed.
   
4. 
   Bybee,Joan; Perkins, Revere; and Pagliuca, William. The Evolution of Grammar, Univ. of Chicago Press, 1994, pp.192-199
   
5. 
   The forms are given as in §85 and in §84 2 of Dansk grammatik (in Danish) by Niels Nielsen, Gleerups förlag, 1959, but with modernised orthography.
   
6. 
   The forms are given as in §77 and in §83 h) of An introduction to modern Faroese by W. B. Lockwood, Thórshavn, 1977.
   
7. 
   Obsolete or dialectal, confused with and replaced by dare (OED, s.v. †tharf, thar, v. and dare, v.1).
   
8. 
   Ian Jacobs. English Modal Verbs. August 1995
   
9. 
   Sakoda, Kent, and Jeff Siegel, Pidgin Grammar, Bess Press, 2003.
   
10. 
    Alexander, W. D., Introduction to Hawaiian Grammar, Dover Publ., 2004
    
11. 
    a b Li, Charles N., and Sandra A. Thomson, Mandarin Chinese: A Functional Reference Grammar, 1989.