While other auxiliary verbs are used for expressing actions or helping main verbs in expressing actions, modals are special auxiliary verbs used to express opinions on an action, or possibility of an action happening.
Unlike other auxiliary verbs, modals can never be used as main verbs in any sentence. Also, many modals have present and past forms (can, could), (shall, should) etc., but the meanings and applications may completely vary.
“Can” is used to express the ability of someone to perform an action in the present time. “Could” is used to express the ability of someone to perform an action in the past. In negative sentences, they follow this structure — “can/could” + “not”.
For Example
Important
“Can/Could” is often used to indicate probability or the possibility of an action happening in the future. In such cases, “could” is more often used than “can”.
For Example
The “can/could” structure is also used to ask for permission. In such cases, the “can/could” is used in question form. “Could” is a more polite form of asking for permission than “can”.
When giving permission, “can” is more often used than “could” and they are used in sentences. On the other hand, making questions with “can’t” increases the persuasion of the speaker.
Can you lend me your bike? (Asking for permission)
Yes, you can. (Giving permission).
Could I send you my resume’? (Asking for permission)
Yes, you can. (Giving permission).
Can’t you help me this one time? (Persuasive request)
Couldn’t you convince him to lend me his car? (Persuasive request)
Suggesting an idea
“Could” is also used to suggest an idea to someone.
For Example
Be able to
“Be able to” has a similar use as that of “can/could”. It is used to express the ability of someone to perform an action in the present time. In these cases, these are the structures they follow −
“Be able to” is also used to express the ability of someone to perform an action in the past. In these cases, these are the structures they follow −
Important
In negative cases, “be able to” uses a “not” after the “be”-form used in the sentence. In future sentences, “be able to” is used as it is with “will” before it. In case of negative future sentences, “be able to” uses “not” between “will” and “be able to”.
For Example
The “May/might” form is used to express the probability of an action happening in the future. In this case, “may” is considered to be a surer guess than “might”. They are also used in question-form to take permission.
Taking permission using “may/might” is considered to be far more polite than “can/could”.
For Example
Important
In some cases, “may/might” is also used to make suggestions. In these cases, “may/might” are often used with “as well” −
For Example
“Shall” and “should” might appear like present-past forms but have completely different applications. “Shall” is used as a substitute of “will” in sentence form while discussing formal matters, and is used as a way to make an offer in the question form.
Important
“Shall” is used more commonly with “I/We”.
For Example
Important
“Should” form is used to make suggestions, advices, and also share opinions. It is also used to sometimes give a future estimate of an event happening as per expectation. Two slightly more persuasive forms of “should” are “ought to” and “need to” where “need to” being the most persuasive of the three.
For Example
“Need to” is used to describe an action that is necessary to be carried out. When used in negative sentences, “need to” and “ought to” replace their “to” with “not”.
Important
The classical negative form of “ought to” is “ought not to” and this form is still in use, so readers should note the options before answering questions related to this.
Another important point is that “ought to” follows the same grammatical rules as “need not” but its meaning is the same as “should” and is treated as a replacement of “should”.
For Example
“Must” is used when the action is a high requirement. “Have to” is an extreme form of “must” where the action is compulsory. “Must” is also used to make a logical prediction, make a warning, and to persuade someone to conduct an action.
When used in negative sentences, “must” becomes “must not” and “have to” becomes “don’t have to” with “I, we, you, they” and plural nouns, and “doesn’t have to” with “he, she, it” and other singular nouns.
For Example
“Will” is used to speak of an action that is going to happen in the future. On the other hand, “would” is used differently; its usage is in recalling some incident that used to happen often in past. “Would” in this context, functions just like “used to”.
For Example
Important
“Will” and “Would” in the question form are used to make polite requests.
For Example