I have a question I hope you can help me with.

In the following sentences what is the form of the main verb used:

1) You ought to give her a ring.
2) You may go out tonight.
3) She must have told him about the affair.

Hello, Jphn Emotion: smile

In all three sentences, the main verbs are preceded by what is called a "modal verb". Modals are are a special type of verbs in English, used to conver certain meanings such as prediction, possibility, permission, ability, etc.

1. The main verb is "give", and the form used in the sentence is the infinitive (the infinitive is the verb as you find it in a dictionary entry, it's the "base" form of the verb). "ought to" is what some authors call a pseudo-modal. Its meaning is similar to that of "should".

2. The main verb is "go" (out). Here, again, the form used is the infinitive. It is difficult to tell here what the exact meaning of the modal "may" is, since the sentence is isolated. But it can mean:
a. permission: She is allowed to go out tonight.
b. deduction/prediction: She's called a taxi, so perhaps she'll go out tonight
c. possibility: She doesn't have to get up early, so it's possible that she will go out tonight.

3. The main verb is "tell", and the form used in this sentence is the "past participle" of the verb (accompanied by "have" to form a perfect tense). The modal "must" suggest deduction or probability.
"It is possible that she has told him about the affair."

I hope it helps.

1) ought to
 miriam's reply was promoted to an answer.