> Njáll sagði að það var skylt.
> Njál said that that was right.

Or more precisely, that that was his right; he was entitled (to that).

> vita hversu þú vilt vera láta."
> to know how you want to let be.'

Or "...to cause to be", i.e. what do you want done, what do you wish
me to do.

> Gunnari ef eg væri hér á landi og mun hann það vitað hafa er hann

vissi dauða sinn fyrir."

> Aðra skóggangssök sel eg þér á hendur Starkaði er hann hefir höggvið

í skógi mínum á Þríhyrningshálsum

As well as meaning literally "when", 'er' can introduce an explanation
of circumstances, "since", "seeing as", "given that". Gordon's
glossary cites these examples under that heading:

ok er þat stórr atburðarhlutr fyrir guði, er vér erum kristnir sjálfir
and that is a matter of grave responsibility before God, since we are
Christians ourselves

ok enn þreifar hann um fleskit, ok finnr at þar var stungit í sverði,
ok kendi at hjöltunum, er myrkt var í hauginum
and again he feels the bacon, and finds that there was a sword stuck
in it, and recognised it by the 'hilt' (pommel and guard), since it
was dark in the mound

LN