Traditionally can is used to show:
 Ability:
An English person who can speak even one other language fluently is rare.
 Possibility:
Relapse can occur at any time.
May is normally used to show:
 Permission:
Candidates may enter for both examinations, if desired.
 Possibility:
It may cause pain, but often there are no symptoms.
Today can is increasingly used to show permission:
Mum, can I leave it?
Using can in this way is accepted as standard English, although many people still use may in more formal situations.
For Scientific english editing and Medical Writing Services visitwww.manuscriptedit.com