While hark is still used today, hearken is pretty much obsolete — unless perhaps you happen to be an old school preacher. Hearken, like hark, can also mean to look back to something in the past, though there is often a critical sense to the word. To hearken back to an old memory or event is usually to do it in a way that may be annoying to other people or perhaps even damaging or self-defeating to the one doing the hearkening. Frequently spelled harken in the US.