[Avodah] Feminine plural imperative

[Zev Sero via Avodah]

What is the standard Biblical suffix for the feminine plural imperative? 
  -n, or -na?   I had always thought it was -na, as in "ts'ena ur'ena", 
and that "kiren lo" in last week's parsha is an exception.  But someone 
pointed out to me that there's also "vahamiten" in the same parsha, and 
claimed that in the language of the Chumash this is the standard form, 
but it had changed by the time Shir Hashirim was written.  Can anyone 
shed light on this?

-- 
Zev Sero                May 2017, with its *nine* days of Chanukah,
zev at sero.name           be a brilliant year for us all