Can, Could, Be able to Can and could are modal auxiliary verbs. Be able to uses the verb "to be" as a main verb. It is not an auxiliary verb, but we look at it here for convenience. Can Can is an auxiliary verb, a modal auxiliary verb. We use "can" to: * talk about possibility and ability * make requests * ask for or give permission Structure of Can subject + can + main verb The main verb is always the bare infinitive (infinitive without "to"). subject auxiliary verb main verb + I can play tennis. - He cannot play tennis. can't ? Can you play tennis? Notice that: * Can is invariable. There is only one form of can. * The main verb is always the bare infinitive. Use of Can can: Possibility and Ability We use can to talk about what is possible, what we are able or free to do: * She can drive a car. * John can speak Spanish. * I cannot hear you. (I can't hear you.) * Can you hear me? Normally, we use can for the present. But it is possible to use can when we make present decisions about future ability. A. Can you help me with my homework? (present) B. Sorry. I'm busy today. But I can help you tomorrow. (future) can: Requests and Orders We often use can in a question to ask somebody to do something. This is not a real question - we do not really want to know if the person is able to do something, we want them to do it! The use of can in this way is informal (mainly between friends and family): * Can you make a cup of coffee, please. * Can you put the TV on. * Can you come here a minute. * Can you be quiet! can: Permission We sometimes use can to ask or give permission for something: A. Can I smoke in this room? B. You can't smoke here, but you can smoke in the garden. (Note that we also use could, may, might for permission. The use of can for permission is informal.) Could Could is an auxiliary verb, a modal auxiliary verb. We use "could" to: * talk about past possibility or ability * make requests Structure of Could subject + could + main verb The main verb is always the bare infinitive (infinitive without "to"). subject auxiliary verb main verb + My grandmother could speak Japanese. - She could not speak Chinese. couldn't ? Could your grandmother speak Japanese? Notice that: * Could is invariable. There is only one form of could. * The main verb is always the bare infinitive. Use of Could could: Past Possibility or Ability We use could to talk about what was possible in the past, what we were able or free to do: * I could swim when I was 5 years old. * My grandmother could speak seven languages. * When we arrived home, we could not open the door. (...couldn't open the door.) * Could you understand what he was saying? We use could (positive) and couldn't (negative) for general ability in the past. But when we talk about one special occasion in the past, we use be able (positive) and couldn't (negative). Look at these examples: Past General Specific Occasion + My grandmother could speak Spanish. A man fell into the river yesterday. The police were able to save him. - My grandmother couldn't speak Spanish. A man fell into the river yesterday. The police couldn't save him. could: Requests We often use could in a question to ask somebody to do something. The use of could in this way is fairly polite (formal): * Could you tell me where the bank is, please? * Could you send me a catalogue, please? Be able to Although we look at be able to here, it is not a modal verb. It is simply the verb "to be" plus an adjective (able) followed by the infinitive. We look at "be able to" here because we sometimes use it instead of "can" and "could". We use "be able to": * to talk about ability Structure of Be able to The structure of be able to is: subject + be + able + infinitive subject be main verb able adjective infinitive + I am able to drive. - She is not able to drive. isn't ? Are you able to drive? Notice that be able to is possible in all tenses, for example: * I was able to drive... * I will be able to drive... * I have been able to drive... Notice too that be able to has an infinitive form: * I would like to be able to speak Chinese. Use of Be able to be able to: ability 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: 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. ............