Learn Icelandic: Hann talar íslensku sæmilega. - He speaks Icelandic reasonably well.

The base word for the Icelandic language is íslenska, which is a feminine noun. The verb talar (speaks) requires the language being spoken to be in the accusative case. For feminine nouns ending in -a, the accusative ending changes to -u.

How is the adverb sæmilega (reasonably well) formed?

It comes from the adjective sæmilegur (decent or reasonable). In Icelandic, you can often turn an adjective into an adverb describing how an action is done by replacing the masculine ending -ur with -a or -lega. This is very similar to adding "-ly" in English.

Do we ever add the definite article to languages, like saying "He speaks the Icelandic"?

No, just like in English, we leave the definite article off when talking about speaking a language. You simply say Hann talar íslensku, never Hann talar íslenskuna.

Does the verb tala mean both "speak" and "talk"?

Yes! While English distinguishes between speaking a language and talking to someone, Icelandic uses the verb að tala for both situations.