Learn Icelandic: Gamli pappírinn gulnar alltaf mikið. - The old paper always yellows a lot.

The verb gulnar (infinitive: gulna) is formed by taking the root of the adjective gulur (yellow) and adding the suffix -na. This suffix creates an inchoative verb, which simply means a verb that describes a change of state—in this case, 'to become yellow'.

Can I use gulna to say someone purposefully made the paper yellow?

No. Verbs formed with the -na suffix are almost exclusively intransitive, meaning they do not take a direct object. They describe a subject undergoing a change by itself (the paper yellows). To say you made something yellow, you would generally use an entirely different verb or a phrase like gera eitthvað gult (to make something yellow).

Why is the adjective gamli used here instead of the dictionary form gamall?

Whenever an adjective comes right before a noun with a definite article (like pappírinn), the adjective must be in its weak form. The weak nominative masculine form of gamall is gamli. If it were indefinite, you would use the strong form: gamall pappír ('old paper').

Is mikið an adjective or an adverb here?

Here, mikið acts as an adverb modifying the verb gulnar, meaning that it yellows 'a lot' or 'greatly'. While mikið is also the neuter singular form of the adjective mikill (much/great), using it as an adverb to describe the degree of an action is extremely common in Icelandic.