APA (7th ed.) Citation

Willis, R., & Szkutnik, L. L. (1968). Helping verbs. Longman.

Chicago Style (17th ed.) Citation

Willis, Roger, and L. L. Szkutnik. Helping Verbs. London: Longman, 1968.

MLA (9th ed.) Citation

Willis, Roger, and L. L. Szkutnik. Helping Verbs. Longman, 1968.

Warning: These citations may not always be 100% accurate.