1. Aarts, Bas and Sean Wallis. 2006. The Diachronic Corpus of Present-Day Spoken English (DCPSE). Survey of English Usage, UCL, London. https://www.ucl.ac.uk/english-usage/projects/dcpse (28 February 2021).
2. Biber, Douglas, Stig Johansson, Geoffrey Leech, Susan Conrad and Edward Finegan. 1999. Longman grammar of spoken and written English. Harlow, Essex: Longman.
3. Bybee, Joan, Revere Perkins and William Pagliuca. 1994. The evolution of grammar: Tense, aspect, and modality in the languages of the world. Chicago: The University of Chicago Press.
4. Coates, Jennifer. 1983. The semantics of the modal auxiliaries. London: Croom Helm
5. Croft, William D. and Alan Cruse. 2004. Cognitive linguistics. Cambridge: Cambridge University Press.