Learn Swedish: Metaforen är rolig att tolka. - The metaphor is fun to interpret.

In a standard sentence like Det är roligt att tolka metaforen, the adjective is roligt because the dummy subject is the neuter word det. But in this sentence, the object of the action (metaforen) has been moved to the front to become the subject. Since en metafor is an en-word, the adjective rolig must agree with it.

What if the subject were plural, like metaforerna?

The adjective agreement rule still applies perfectly in this construction. If the subject is plural, the adjective takes the plural ending -a. You would say Metaforerna är roliga att tolka.

Can I use this same grammatical structure with any adjective?

This structure is mainly used with evaluative adjectives that describe how difficult, easy, fun, or boring an action is. Common adjectives you will see used this way include svår, lätt, enkel, tråkig, and farlig.

Does the verb tolka refer to literary interpretation or language interpretation?

It refers to both! You can use tolka for interpreting a text, a metaphor, or a piece of music, as well as for translating spoken language live. A spoken language interpreter is simply called en tolk.