When do we you an apostrophe before a vowel? Why do we use it in "дев'ять" but don't use in "десять"?
Yes, apostrophe is used only after specific letters with some exceptions and is never used after certain letters like С for example
Roughly speaking, десять = [десьать], дев’ять = [девйать]
(of course it's wrong to spell like that, only for transcription)