an aid

Very rough and probably not complete:

Sometime - at an unspecified time in the past -

James Smith, sometime professor of Latin at Oxford University (James Smith, former professor etc)
Her sometime boyfriend - her former boyfriend

sometimes - occasionally -

Sometimes I go swimming after work.

some time -

(1) at a time in the future not yet decided -

Some time I must tidy up the garden. (But I don't know when.)

(2) with 'at' - a time in the past not specified -

At some time in his life he visited India.

(3) an unspecified amount of time:

I spent some time with Mary yesterday

maybe more...

some times - an unspecified number of occasions or periods.

There are some times in my life I do not like to remember.