We use be able to to express ability. "Able" is an adjective meaning: having the power, skill or means to do something. If we say "I am able to swim", it is like saying "I can swim". We sometimes use "be able to" instead of "can" or "could" for ability. "Be able to" is possible in all tenses—but "can" is possible only in the present and "could" is possible only in the past for ability. In addition, "can" and "could" have no infinitive form. So we use "be able to" when we want to use other tenses or the infinitive. Look at these examples:

• I have been able to swim since I was five. (present perfect)

• You will be able to speak perfect English very soon. (future simple)

• I would like to be able to fly an airplane. (infinitive)

Have To (objective obligation)

We often use have to to say that something is obligatory, for example:

• Children have to go to school.

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:

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.

In each of the above cases, the obligation is not the subject's opinion or idea. The obligation is imposed from outside.

We can use "have to" in all tenses, and also with modal auxiliaries. We conjugate it just like any other main verb. Here are some examples: