CAN AND COULD
The verb “can” has one form for all singulars and plurals and occurs only in the present tense. It does not require the use of any additional auxiliary verb, as the questions are formed through the inversion, and negatives through the addition of the word “not.”
The full form of negative form is written jointly: can + not = cannot.
After “can” we put an infinitive without a “to.”
We use this verb to express that something is possible or impossible:
|affirmative sentence||Fast driving can be very dangerous.|
|interrogative sentence||Can smoking be good for your health?|
|negative sentence||He can’t be home already. I saw him in town five minutes ago.|
We use this verb to determine someone’s abilities and skills:
|affirmative sentence||She can speak French and Spanish.|
|interrogative sentence||Can you run fast?|
|negative sentence||I can’t drive.|
The verb “could” also have only one form; it requires the use of the second verb in an infinitive without “to“ and forms questions and negatives without any additional auxiliary verbs.
“Could” is past form of “can”,
We use “could” to describe past skills, abilities, lack of abilities and (im)possibilities:
|affirmative sentence||I could swim when I was 5.|
|interrogative sentence||Could you answer all the questions in the test?|
|negative sentence||She couldn’t find the keys.|
The verb “could” is used to express requests and asking for permission. It is a more polite form of the verb “can.”
Excuse me. Could you tell me how to get to the nearest post office?
Could you show me the way out, please?
Could you lend me your bicycle?
The modal verb “should” express advice. It is also used when speaking of what seems appropriate in a given situation:
You look so pale. You should see a doctor.
I think I should look for a better job.
The verb that follows “should” is in an infinitive without to:
|affirmative sentence||You should visit them one day.|
|interrogative sentence||When should I start?|
|negative sentence||You shouldn’t believe everything you see on the Internet.|