> inn í fjarðarkjafta þá er þar voru næstir
> in into a firth mouth then where there were nearest

Lit. "in into those firth-jaws those [i.e. those firth-jaws] which
there were nearest"
More idiomatically: "into the mouth of the nearest firth"

'fjarðarkjaptr', masculine, lit. "firth's jaw". 'fjarðarkjapta' is
accusative plural'.

'þá' is the masc. acc. pl. demonstrative pronoun, agreeing with
'fjarðakjapta', and used here to support the relative particle: 'þá
er' "(the/those) ... which".

'þar ... næstir' "nearest (to there)"

> og að höfða þeim er þar gekk fram. Hann var allur skógi vaxin.
> and to those headlands which went forth. It was all grown with

forest.

'höfða' is singular (pl. would be 'að höfðum þeim'); and compare
'hann' in the next sentence, and singular + def. art. 'höfðanum'
later. I guess 'vaxin' is a typo for 'vaxinn'.

> Þá leggja þeir fram skip sitt í lægi
> Then they reefed sail? from their ship into anchorage

"Then they brought their ship (forwards) into the anchorage"

> til þangað
> to (them) at once

"thither to them". 'til' and 'þangað' express the same idea really:
where they're going.