You just suppose that the almost red thingy is an en-word. You could be talking about the sun for example:
Titta på solen! Den är nästan röd.
So if the red thing were a bright brown egg, it would be "Det är nästan röd" right?
And which would I use if I want to say "They are almost red"... Would it be det, or detta or something?
Okay, thank you! I suppose I should go review the adjectives lesson again. I think I must be over-thinking them and confusing myself. xD
To reorder the English sentence like that, you would need a comma: "It is red, almost". This probably tells you that you are rewriting the sentence, rather than translating it. It deemphasizes the adverb, or makes the adverb an afterthought. I don't know if you could get that same effect from: "Den är röd, nästen".
So then, would "It is almost dead" be "Det är nästan dött?" (Edited 2017/04/09)
I did read the other comments, but I still don't know why it is 'den.' Is it because we are assuming that the "it" is an en- word?
It could have been det but this speaker apparently knows he is referring to a en word which was probably introduced in the previous sentence. Also, that context is why the speaker used röd, instead of rött.