Learn Icelandic: Muna margir íbúar töluna? - Do many inhabitants remember the number?

Muna is the infinitive, but it is also the "they" form (third-person plural) in the present tense. The subject here is margir íbúar (many inhabitants). Muna is a slightly unusual verb: its singular forms are ég man, þú manst, hann man, while its plural forms are við munum, þið munið, þeir/þær/þau muna.

Why is the word for the number töluna instead of talan?

The verb muna (to remember) requires its object to be in the accusative case. The base word is the feminine noun tala. In the accusative case, it becomes tölu, and adding the feminine definite article for "the" gives us töluna.

Why does the first vowel change from 'a' in tala to 'ö' in töluna?

This happens because of a common Icelandic vowel change called the U-shift. Whenever an 'a' in the stem of a word is followed by a 'u' in the ending (like the accusative ending in tölu), the 'a' changes to 'ö'. The 'ö' remains even after the definite article -na is attached.

Why does the sentence start with the verb Muna?

In Icelandic, you form a yes/no question by putting the verb at the very beginning of the sentence. Icelandic does not use helper verbs like "do" or "does", so the sentence literally reads "Remember many inhabitants the number?".