Yes

There are two ways to say “yes” in Icelandic:

When answering “yes” to a question, you use já.
When answering “yes” to a negative question, you use jú.

“Jú” is used any time a question includes the word “ekki”. This is comparable to the English “Yea it is!” or “Yuh huh!” which are used when someone expresses doubt.

Example:

Er þetta góður hamborgari? Já.
Er þetta ekki góður hamborgari? Jú.

There’s only one “no”:

Er þetta góður hamborgari? Nei.
Er þetta ekki góður hamborgari? Nei.

Yeah sure

You can duplicate the words to be more casual. “Já já” means “yeah sure” like you’re less committed to the answer. This lack of committment can be used to show politeness, like you’re saying “sure no problem”.

já já = yeah sure (more casual than just “já”)
jú jú = yeah sure (responding to a negative question, more casual than just “jú”)
nei nei = nah (more casual than just “nei”)

1|Er þetta gott?
2|Já já, þetta er ágætt.
1|Er þetta ekki gott?
2|Jú jú, þetta er ágætt.
1|Ætlar þú að koma á eftir?
2|Nei, ég ætla ekki að koma á eftir.

What did you just say?

Ha? is equivalent to the English “Huh?” or “Hm?” but is used way more than in English. It means “What did you just say?”, but it’s not rude at all. You say “ha?” when:

You didn’t hear what the other person said
You just can’t believe what the other person said

The manner in which it is said is a bit harsh, and therefore many students feel that it sounds agressive or rude. That’s not the case at all, “ha?” is a very normal word and is not rude at all. It is used both between friends and in professional settings.

Remember that you don’t say “Could you repeat that?” or “Would you mind talking a bit slower?”, you just say “Ha?”.

Recommendations

Listen to the 1992 Eurovision song Nei eða já (lyrics). The only parts of the lyrics you should focus on are “Nei eða já” (No or yes).