APA aipamena

Willis, R. (1975). Helping verbs. Longman.

Chicago Style aipamena

Willis, Roger. Helping Verbs. London: Longman, 1975.

MLA aipamena

Willis, Roger. Helping Verbs. Longman, 1975.

Kontuz: berrikusi erreferentzia hauek erabili aurretik.