From your friendly grammar guru:
The ‘-st’ ending on some words is an archaic form that mostly disappeared after the 18th century (in Shakespeare you get things like “why hatest thou me?’ etc).
English English – as opposed to American English – has sometimes retained these forms in formal usage, just as American English has held on to ‘gotten’ where we would say ‘got’. Most style guides now advise using ‘while’ instead of ‘whilst’ and ‘among’ as opposed to ‘amongst’. So, in my own writing, I’ve abandoned the older forms.
Regular readers will know that my background is in English teaching and literacy, as my JAM anthem celebrates:
http://geoffbarton.co.uk/files/_IDs/Nice-Knowing-Geoff.mp3
You did ask!