The modal verb can has two forms. The original form of the verb (present tense) is can, and the past tense is could.

The basic usage of can(could) is:

1) means can. Such as:

I can lift this stone. I can lift this stone. (Showing physical ability)

Can you use chopsticks? (Showing skills)

I can see him tonight. I can see him tonight. (meaning possible)

The can tense is synonymous with the phrase be able to, but the latter can be used in various tenses.

The can expression can be used in various sentence patterns when possible. For example:

She can play a few simple tunes on the piano. She can play a few simple tunes on the piano. (Affirmative sentence)

Can you write with your left hand? Can you write with your left hand? (Interrogative sentence)

I can not promise you anything. I can not promise you anything. (Negative sentence)

To express the future tense, use the phrase will be able to. Such as:

I shall be able to earn my own living soon. I will be able to stand on my own feet soon.

He says he’ll be able to be home for Christmas.

Could mainly refers to the past. Such as:

I said that I could go. I said I could go.

Could the boy read before he went to school? Could the boy read before he went to school?

Was or were+able can also mean that he was able in the past. Such as:

I was able to help you yesterday. I was able to help you yesterday.

But could can also often refer to the present or the future. Such as:

You could phone her, I suppose. I suppose you can call her.

The river could easily overflow, couldn't it? The river could easily overflow, couldn't it?

If could is used with the perfect form of the verb's base form, it refers to an unfulfilled action in the past . Such as:

She could have explained the mystery. She could have explained the mystery. (Actually not explained)

2) Possible. For example:

The moon cannot always be at the full. The moon cannot always be at the full.

If it’s raining tomorrow, the sports can take place indoors. If it rains tomorrow, the sports can take place indoors.

can can refer to the present or the future when possible. For example:

You can perhaps obtain a dog from the Dogs’ Home. (Affirmative sentence)

What can he mean? What can he mean? (Interrogative sentence)

We can't use the indefinite article with this noun. We can't use the indefinite article with this noun. Use the indefinite article before nouns. (Negative sentence)

When expressing possibility, could can refer to the past. Such as:

He said he couldn’t agree more. He said he couldn’t agree more.

Could can also refer to the present or the future, expressing the subjunctive mood. Such as:

It could be my mother. It could be my mother.

He could arrive tomorrow. He could arrive tomorrow.

Could is used with the perfect tense to refer to unfulfilled actions in the past. Such as:

It could have been seen from here if it had not been so dark. If it were not so dark, you might have seen it from here. (actually invisible)

3) means permission.

For example:

You can borrow my bike tomorrow. You can borrow my bike tomorrow.

To express this meaning, can is used in questions to express requirements, and when used in negative sentences it expresses disallowance. Such as:

Can you lend me a hand? Can you lend me a hand?

This sort of thing can't go on! This kind of thing can't go on!< /p>

could refers to the present tense and is only used in questions to express more euphemistic requests. For example:

Could I interrupt a moment? Can I insert a sentence?

The allowed usage of this kind of table can be used in noun clauses whose main clause is the past tense. At this time, could refers to the past, such as:

Father said I could swim in the river. Dad said I could swim in the river.

4) Express doubts. For example:

Can it be true? Can it be true?

When expressing this meaning, can is only used in general questions and has an emotional tone.

can is used in general expressions. For example:

Can he really be ill? Can he really be ill?

Can is used in the progressive tense to express the future. For example:

Can he be making the investigation all alone? Can he be making the investigation all alone?

Can is used in the perfect tense to express the past. For example:

Can she have told a lie? Can she lie?

Can can also express a continuous action when used in the perfect tense or the perfect progressive tense. Such as:

Can she really have been at home all this time? Can she have been waiting for us so long? How long?

Could can also be used in these ways in the present tense, but it implies a more uncertain meaning. Such as:

Could it be true?

Could she be telling lies?

Could he have said it?

Could he have been at home all this time?

Could she have been waiting for us so long?

5) can't means unlikely. Such as:

It can’t be true. That is unlikely to be true.

can’t be emotional. Such as:

He can’t be really ill. It is unlikely that he is really ill. (referring to now)

She can’t be telling lies. She is unlikely to lie. (referring to now)

He can’t have said it. It is unlikely that he would say such a thing. (referring to the past)

She can’t have been at home all this time. It is unlikely that she has been at home all the time. (referring to the past)

She can’t have been waiting for us so long. She can’t have been waiting for us so long. (referring to the past)

could can also be used in these ways, but in a more euphemistic tone. Such as:

It couldn't be true.

She couldn't be telling lies.

He couldn't have said it.

She couldn't have been at home all this time.

She couldn't have been waiting for us so long.

6) can and could are used in special questions , emotional, expressing surprise, confusion, etc.

Such as:

What can(could) he mean? What may he mean?

What can(could) he be doing? What may he be doing?

What can(could) he have done? What can(could) he have done?

Where can(could) he have gone to? Where can(could) he have gone to?

7 ) Comparison of can and could: can and could represent able and possible, can represents real and could represents unreal. Such as:

He can speak English. He can speak English. (meaning able)

He could speak English if necessary. He could speak English if necessary.

You can get the book from the library. You can borrow this book from the library. (meaning possible)

You could get the book from the library if necessary. You can borrow this book from the library if necessary.

When expressing permission and speculation, could has a more euphemistic tone and a less certain meaning. For example:

Can I use your pen? Can I borrow your pen? (indicating permission)

Could I use your pen? (more euphemistic)

Can it be true? Could that be true? (speculation)

(69)Could it be true? (less certain)

(70)It can' t be true. That is unlikely to be true. (unlikely)

(71) It couldn’t be true. (relatively uncertain)

8) Used in fixed idioms. Such as:

She can’t help crying. She couldn’t help crying.

He couldn’t help laughing.

I can’t but ask him about it.

They couldn’t but refuse him.