Learn Icelandic: Stóllinn er harðari en sófinn. - The chair is harder than the sofa.

Harðari is the comparative form of the adjective harður (hard). To make a regular adjective comparative (meaning "-er" or "more"), you take the stem of the word by dropping the masculine -ur ending, and add -ari. So, harð- + -ari gives us harðari (harder).

Doesn't the word en usually mean "but"?

Yes, en is the most common word for "but" in Icelandic. However, when it is used to connect two things being compared, usually right after a comparative adjective like harðari (harder), it acts as the word "than".

If I were comparing feminine or neuter nouns instead of masculine ones, would harðari change its ending?

No, and that makes things much easier! Unlike base adjectives which change depending on whether a noun is masculine, feminine, or neuter, the comparative -ari form does not change for gender. Harðari stays exactly the same no matter what you are comparing.

Why do stóllinn and sófinn have slightly different "the" endings if they are both masculine?