MODAL: can, could, may, might, must, be able to, has/have/had
Formula: subject + modals + main verb
Can talk about possibility and ability, to make requests
Could talk about Past Possibility or Ability, to make requests
Requests
Be able to talk about ability Notice that be able to is possible in all tenses
May is Possibility, necessity
Must is a modal auxiliary verb.
Have to is NOT an auxiliary verb (it uses the verb have as a main verb).
Have to (objective obligation) We often use have to to say that something is obligatory. 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.
Must (subjective obligation) We often use must to say that something is essential or necessary In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective. In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside. We cannot use must to talk about the past. We use have to to talk about the past
Must not/Mustn't (prohibition) We use must not to say that something is not permitted or allowed, Must not expresses prohibition - something that is not permitted, not allowed. The prohibition can be subjective (the speaker's opinion) or objective (a real law or rule). We can use must not to talk about the present or the future:
We cannot use must not to talk about the past.
Example:
All applicants must take an entrance exam
We have to obey the law
Formula: subject + modals + main verb
Can talk about possibility and ability, to make requests
Could talk about Past Possibility or Ability, to make requests
Requests
Be able to talk about ability Notice that be able to is possible in all tenses
May is Possibility, necessity
Must is a modal auxiliary verb.
Have to is NOT an auxiliary verb (it uses the verb have as a main verb).
Have to (objective obligation) We often use have to to say that something is obligatory. 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.
Must (subjective obligation) We often use must to say that something is essential or necessary In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective. In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside. We cannot use must to talk about the past. We use have to to talk about the past
Must not/Mustn't (prohibition) We use must not to say that something is not permitted or allowed, Must not expresses prohibition - something that is not permitted, not allowed. The prohibition can be subjective (the speaker's opinion) or objective (a real law or rule). We can use must not to talk about the present or the future:
We cannot use must not to talk about the past.
Example:
All applicants must take an entrance exam
We have to obey the law
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