I cannot tell you how it was;
But this I know: it came to pass
Upon a bright and breezy day
When May was young; ah, pleasant May!
This is a key line, repeated almost exactly in line nine, but with the significant change of ‘how’ to ‘what’. This is a device known as  anaphora , 
 The opening ‘I cannot tell’ is ambiguous. Does this mean the speaker is not allowed to tell, or that she lacks the ability or understanding to tell?
May