i Register
In some senses, erst is marked as obsolete, archaic, poetic. Watch for register when choosing this word.
adj
First.
adv
First of all, before (some other specified thing).
Consider what I erst have been and what thou seest me now:
Sooner (than); before.
Thenne he sente the varlet ageyne and bad hym telle Kyng Mark that I wille come as soone as I am hole / for erste I maye doo hym noo good "Then he sent the varlet again, and bade him: Tell King Mark that I will come as soon as I am whole, for erst I may do him no good. "
Formerly, once, erstwhile.
When lofty trees I see barren of leaves Which erst from heat did canopy the herd