Expressing ability

Usage Modal Verb Example
Have the ability to do something (positive) can / to be able to Many people can speak more than one language.

Many mothers are able to manage a family and a full time job.

Have the knowledge which gives the ability to do something (positive) know how to Most children know how to use computers.
Not have the ability to do something (negative) cannot / to (not) be able to Many native English speakers cannot speak a second language.

Some graduates are not able to find a job, despite their studies.

Not have the knowledge which gives the ability to do something (negative) to (not) know how to A large proportion of older people do not know how to use computers

IMPORTANT NOTE:

‘Know how to’ cannot be used to replace ‘able to’ or ‘can’ in the various tenses where no knowledge is involved. The following sentence is incorrect:

In the past, children knew how to walk to school on their own without any problems; these days, parents worry about their safety.

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