IELTS, TOEFL and TOEIC writing - how do we use can and could?

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How do we use can and could in exam writing?

Can and could are modal verbs . Many students are taught that can is the present tense and could is the past tense, but using these verbs is much more complicated.

Can means "to be able to do something", or "possibility".  Examples:

I can speak three languages - (I have the ability to speak three languages).
He is very unwell and can't come to work - (He is unable to be present at work).
We can see the stars when we are in the mountains - (The height of the mountains makes it possible for us to see the stars).

Could is used in polite forms of questions and answers for possibility or conditions. It is also used to express ability (see can) in the past.

Could you pass me the salt, please? - (A polite form of asking for the salt).
If I had more time, I could travel more. - (This is the second conditional - it is possible, but not probable.)
He could do the work, of course, if he chose to concentrate. - (The second conditional again. He has the ability to do the work, but he needs to concentrate.
Women could not vote in France before World War II. - (Women were not able to vote before 1945).

Choose the correct word (can or could) for the following sentences:

1. Heavy industry ____________________ help the economy grow by hiring new workers.
2. ____________________ foreign students enroll in classes without a student visa?
3. If the antibiotic is given soon enough, it _____________________ stop the infection.
4. If Napoleon had had a stronger navy, he ______________________ have invaded England.
5. _________________ you show me which building is the Science Centre?
6.The modern dog ________________ descend from wolves which followed hunters and gatherers.

1. Heavy industry can help the economy grow by hiring new workers.
2. Can foreign students enroll in classes without a student visa?
3. If the antibiotic is given soon enough, it can stop the infection.
4. If Napoleon had had a stronger navy, he could have invaded England.
5. Could you show me which building is the Science Centre?
6.The modern dog could descend from wolves which followed hunters and gatherers.