Learn Icelandic: Verðið er gott en gæðin eru slæm. - The price is good but the quality is bad.

You would use the comparative forms of these adjectives: Verðið er betra en gæðin eru verri. Notice that betra is the neuter singular comparative form to match verðið, while verri is the plural comparative form used to match the plural noun gæðin.

Why do we use gott and slæm instead of the dictionary forms góður and slæmur?

Adjectives always change their endings to match the gender and number of the noun they describe. Verðið (the price) is neuter and singular, so we use the neuter singular form gott. Gæðin (the quality) is a neuter plural word, so we use the neuter plural form slæm.

In English, we say 'the quality is', but here it says gæðin eru (the quality are). Why is that?

The Icelandic word for quality, gæði, is a neuter plural noun. Even though it translates to a singular concept in English, Icelandic grammar treats it as a plural. Because it is plural, you must use the plural verb eru (are) instead of the singular er (is).

How exactly is 'the' attached in the words verðið and gæðin?