We use "have, has, had" in our everyday speech, therefore it's critical to understand the fundamentals. "Has" is used with the pronouns he, she, and it, and with singular subjects. "Have" is used with the pronouns I, you, we, and they, and with plural subjects. The verb "have, has, had" are used as auxiliary Verbs as well as  main (ordinary) verbs. They also do the function of 'modal verbs'. 'The present tense is indicated by "has" and "have," whereas the past tense is indicated by "had."

This book will help you understand how to use the verbs have, has, and had in English grammar.