Learn Icelandic: Kunnið þið að prjóna?

When kunna is used to mean knowing how to do something, it must be followed by að plus the infinitive verb. This makes it different from other modal verbs you have seen like vilja (to want), which connect directly to the next verb without að.

Could I use getið instead of kunnið to ask 'Can you knit?'?

It would change the meaning. Kunna asks about the learned skill: Do you know how to knit? Geta asks about the physical ability or possibility right now: Are you able to knit? (For example, do you have yarn with you?). Also, remember that geta forces the next verb into the past participle (getið þið prjónað?).

Why does the verb use a 'u' in kunnið when the singular 'you' form is kannt?

Kunna has a vowel shift between its singular and plural forms. In the singular present tense, the vowel is 'a' (ég kann, þú kannt, hún kann). But in the plural, it goes back to the 'u' from the dictionary form (við kunnum, þið kunnið, þau kunna).

If I want to ask just one person, how does this sentence change?