Forms of the verb to have are used to create tenses known as the present perfect and past perfect. The perfect tenses indicate that something has happened in the past; the present perfect indicating that something happened and might be continuing to happen, the past perfect indicating that something happened prior to something else happening. (That sounds worse than it really is!) See the section on Verb Tenses in the Active Voice for further explanation; also review material in the Directory of English Tenses.
To have is also in combination with other modal verbs to express probability and possibility in the past.
- As an affirmative statement, to have can express how certain you are that something happened (when combined with an appropriate modal + have + a past participle): "Georgia must have left already." "Clinton might have known about the gifts." "They may have voted already."
- As a negative statement, a modal is combined with not + have + a past participle to express how certain you are that something did not happen: "Clinton might not have known about the gifts." "I may not have been there at the time of the crime."
- To ask about possibility or probability in the past, a modal is combined with the subject + have + past participle: "Could Clinton have known about the gifts?"
- For short answers, a modal is combined with have: "Did Clinton know about this?" "I don't know. He may have." "The evidence is pretty positive. He must have."
To have (sometimes combined with to get) is used to express a logical inference:
- It's been raining all week; the basement has to be flooded by now.
- He hit his head on the doorway. He has got to be over seven feet tall!
Have is often combined with an infinitive to form an auxiliary whose meaning is similar to "must."
- I have to have a car like that!
- She has to pay her own tuition at college.
- He has to have been the first student to try that.
