Learn Icelandic: Ég get flautað lagið.

The verb geta (can or to be able to) is unique in Icelandic. While most other helper verbs take the infinitive, geta always requires the supine form of the main verb. That is why we use the supine flautað instead of the infinitive að flauta.

How do I build a supine form like flautað?

For many regular verbs ending in -a like flauta, you just replace the final -a with -að. This supine form is the exact same form you use after the verb hafa (to have) to make the perfect tense, such as in ég hef flautað (I have whistled).

What case is lagið in this sentence?

It is in the accusative case because it is the direct object being whistled. The base word is the neuter noun lag (song). For neuter nouns, the accusative form is completely identical to the nominative dictionary form, which is why it looks like it has not changed. The -ið on the end is the attached definite article meaning 'the'.

Is the verb flauta related to the English word 'flute'?

Yes, it comes from the same root! In Icelandic, the verb að flauta usually means to whistle or to honk a car horn. The noun for the musical instrument (a flute) is also flauta.