|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|
‘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.