Learn Icelandic: Málmurinn og glerið eru hrein. - The metal and the glass are clean.

When you have two or more subjects of different genders connected by og (and), Icelandic always defaults to the neuter plural form for the adjective. Since málmur is masculine and gler is neuter, the adjective describing them both must be neuter plural.

I notice hrein is also the feminine singular form of this adjective. Is it feminine here?

No, it just happens to look exactly the same! For most Icelandic adjectives, the neuter plural nominative form is identical to the feminine singular nominative. Here, it is functioning as a neuter plural.

If I wanted to practice the current lesson and say 'These metals and these glasses are clean', how would I do it?

You would use the plural forms of the demonstrative pronoun: Þessir málmar og þessi gler eru hrein. Notice that the adjective hrein stays in the neuter plural because the combined subject is still a mix of masculine and neuter items.

How do the words for 'the' work at the end of málmurinn and glerið?

In Icelandic, 'the' is attached as a suffix rather than being a separate word. Because málmur is a masculine noun, it takes the masculine singular suffix -inn. Gler is a neuter noun, so it takes the neuter singular suffix -ið.