Learn Icelandic: Maðurinn elskar fisk.

Fisk is the direct object (the thing being loved) in this sentence, so it needs to be in the accusative case. For regular masculine nouns that end in -ur like fiskur, you just drop the -ur to make the accusative bare noun.

How would I say "The man loves the fish"?

You would attach the definite article to the end of the accusative word. For a masculine word like fisk, the accusative article is -inn, making it Maðurinn elskar fiskinn. In our sentence, we just have the bare noun fisk, which simply means "fish".

Where does the -inn come from in maðurinn?

Icelandic doesn't have a separate word for "the". Instead, it attaches to the end of the noun. The base word is maður (man). To make it "the man" as the subject of the sentence, we drop the -ur ending and attach the masculine article -inn to get maðurinn.

I have seen elska before. Why is there an -r on elskar here?

Verbs in Icelandic change depending on who is doing the action. Elska is used for "I" (ég elska), but when talking about a third person like "the man", "he", or "she", the verb takes an -r ending to become elskar.