| 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.