Today's topic is have to 

Have To

"Have to" is used to express certainty, necessity, and obligation.

Examples:

• This answer has to be correct. certainty
• The soup has to be stirred continuously to prevent burning. necessity
• They have to leave early. obligation

We use  "Have to" in Present, Past, and Future.

Note that we can use the have to expression in all tenses, for example:
I have to, I had to, I have had to, I will have to

Structure of Have to

Have to is often grouped with modal auxiliary verbs for convenience, but in fact it is not a modal verb. It is not even an auxiliary verb. In the have to structure, "have" is a main verb. The structure is:
subject + auxiliary verb + have + infinitive (with to)
Look at these examples in the simple tense:

	subject	auxiliary verb	main verb have	infinitive (with to)
+	She		has	to work.
-	I	do not	have	to see	the doctor.
?	Did	you	have	to go	to school?

Use of Have to

In general, have to expresses impersonal obligation. The subject of have to is obliged or forced to act by a separate, external power (for example, the Law or school rules). Have to is objective. Look at these examples:

• In France, you have to drive on the right.
• In England, most schoolchildren have to wear a uniform.
• John has to wear a tie at work.

REMEMBER: "Do not have to" vs. "Must not"
"Do not have to" suggests that someone is not required to do something. "Must not" suggests that you are prohibited from doing something.

Examples:

• You must not eat that. It is forbidden, it is not allowed.
• You don't have to eat that. You can if you want to, but it is not necessary.