'Ought to' and 'could' may confuse learners as they both talk about possibilities. In this lesson, we will learn more about them.

"Ought To" vs. "Could" in the English grammar

What Is Their Main Difference?

The main difference between 'ought to' and 'could' is that 'ought to' is a semi-modal verb while 'could' is a modal verb.

Ought to

'Ought to' is a semi-modal verb. Semi-modal verbs sometimes act like modal verbs and sometimes act like a main verb. For example:

It ought to be Harry at the door.

She ought to take care of his sister.

Could

'Could' is also another well-known modal verb. To put it simply, 'could' is the past tense of 'can.' It is used to talk about abilities, request something, etc. For example:

It could be Harry at the door.

The information could be true.

Similarity

Talking about Possibility

We use 'ought to' and 'could' to talk about possible events.

  • 'Could' expresses slight possibilities that we are uncertain about their accuracy:

You could have fallen off the tree.

She could arrive anytime now.

  • 'Ought to' expresses what we assume will happen and we have a higher level of certainty than 'could:'

They ought to be done with the meeting.

She ought to arrive anytime now.

Differences

Talking about Obligations

'Ought to' is used to talk about duties and obligations that are motivated by law, society, someone, or oneself. 'Ought to' is uncommon and its usage has lowered significantly. 'Must' often replaces it when expressing obligations. Have a look:

You ought to drive carefully.

You ought to be home by 9 p.m.

Talking about Abilities

We use 'could' to talk about what we were able to do. These are past abilities that we no longer possess or the person with the ability has been deceased. For example:

My grandmother could knit.

I could swim well when I was younger.

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