not > [it not]

4 But it must doubled be with death of twain?

twain > two

5 For naught for me but death there does remain."

For > Because

6 "O daughter dear," said she, "despair no whit;

daughter > {Girl, maiden; also a term of affectionate address used by any older person to a girl or young woman}

7 For never sore, but might a salve obtain:

never > [there was never a]

8 That blinded god, who has you blindly smitten,