한국말 잘 못해요 (han-gug-mal-jal-mot-hae-yo)

I cannot speak Korean very well

한국말 전혀 못해요 (han-gug-mal-jeon-hyeo-mot-hae-yo)

I cannot speak Korean at all

모르겠어요 (mo-reu-gess-eo-yo)

I don’t understand

미안하지만, 다시 한번 말씀해 주세요 (mi-an-ha-ji-man, da-si-han-beon-mal-sseum-hae-ju-se-yo)

I’m sorry…

I/Me
저 -More formal (becomes 제가 when “I” is the subject of the sentence)
나 - More casual (becomes 내가 when “I” is the subject of the sentence)
Note: Don’t confuse 저/제 & 나/내, because 제 & 내 mean “my”

You
당신 - Most formal (use with stranger or person you don’t know well)
너 - More casual

We 
저희 - More formal
우리 - More casual

Remember, most pronouns are implied in speech, but they are used (: