"Ai" is a common interjection to express pain. e.g.: "Ai meu dedo!" (Ouch my finger!)

I agree with Alexandre. In addition, “Aí” means “there”, or an informal form of “in this case” or “Then”. If someone ask “E aí?” it is a slang for “How are the things doing there?”. (aí as “there”) I have to say that I get confused with “aí” and “lá” as positions in English. “Lá” works very like “there”. It is cool there. É legal lá. It is cool there, where you are. É legal aí/Aí é legal. I have learned that “aí” is also “there” in English. It is quite complicated to me to understand. “Aí’ indicates is the place where the person who you are talking with is. “Lá” is any other place than where you or the person are. Consider this scenario. Paulo is in England, Alex is in Portugal and they are talking about Brazil. -Como estão as coisas aí? Diz Paulo – Aí in this case refers to Portugal only. -Tudo bem - Diz Alex. -Lá no Brasil as coisas não estão muito bem. (“Lá” refers to another place, anywhere where they are not) I’m not sure, but “lá” sounds more like “over there”.