The grammatical descriptions on this page are informed by the following published reference and descriptive grammars. Grammatical facts themselves are not subject to copyright; the scholars who documented them deserve attribution.
Huddleston, R. & Pullum, G.K. (2002). The Cambridge Grammar of the English Language. Cambridge University Press. ISBN 978-0-521-43146-0.
Quirk, R., Greenbaum, S., Leech, G. & Svartvik, J. (1985). A Comprehensive Grammar of the English Language. Longman. ISBN 0-582-51734-6.
Biber, D., Johansson, S., Leech, G., Conrad, S. & Finegan, E. (1999). Longman Grammar of Spoken and Written English. Longman. ISBN 0-582-23725-4.
Carter, R. & McCarthy, M. (2006). Cambridge Grammar of English: A Comprehensive Guide. Cambridge University Press. ISBN 978-0-521-58846-1.
Swan, M. (2016). Practical English Usage. 4th ed. Oxford University Press. ISBN 978-0-19-420243-5.