Ehrman, Madeline.1966. The meanings of the modals in present-day American English . The Hague: Mouton.
2.
Fries, Charles C.1940. American English grammar. New York : Appleton-Century-Crofts.
3.
Hammerly, Hector.1977. The teaching of "ser/estar" by a cognitive audiolingual approach. Hispania60. 305-308.
4.
Hofmann, T.R.1966. Past tense replacement and the modal system. Harvard computational laboratory report to the National Science Foundation on mathematical linguistics and automatic translation. No. NSF -17. Cambridge, Mass.: Harvard University.
5.
Jenkins, Lyle.1972. Modality in English syntax. Bloomington : Indiana University Linguistics Club.
6.
Joos, Martin. 1966. The English verb. Madison, Wisc.: The University of Wisconsin Press.
7.
Keyser, Samuel Jay and Postal, Paul.1976. Beginning English grammar. New York : Harper & Row.
8.
Lee, D.A.1975. Modal 'auxiliaries' in generative grammar - some pedagogic implications. IRAL (International Review of Applied Linguistics in Language Teaching )13. 263-274.
9.
Moulton, William G.1966. A linguistic guide to language learning. New York: The Modem Languages Association .
10.
Seuren, P.A.M.1969. Operators and nucleus; a contribution to the theory of grammar. Cambridge: Cambridge University Press.