I'm trying to determine the exact and complete conjugation of the verb
"Hefja" (hefr, hóf, hófu, hafinn/hafiðr - heave, begin).

From Sweet's Icelandic Primer, hefja, deyja and hlæja are similar to
fara or vaxa, but have weak presents, which looks consistent with the
modern icelandic conjugation:

What I'm really unsure about is:

1- Is the 2nd pers. pl., pres. indic. "hefjið" or "hefið ?

2- Is the root vowel in the past subjunctive "æ" or "œ" (as in fara) ?

Could someone help me ? Thanks in advance !

Gaël Deest