Can I say "That's sucks" when others had a bad event?

Native speaker’s answer
Rebecca
"That sucks" is a very casual, informal way of expressing one's sympathy or condolences for an unfortunate event. The speaker has used it here because he is still young and able to use casual expressions like this, and has treated it lightly to not make his friend feel bad. However, note that this expression is not appropriate for all situations. If it's a serious incident, for example, "that sucks" would not be appropriate - "I'm sorry to hear that" would be a more suitable response. Ex: A: Why were you late to work today? B: There was a huge traffic jam. A: Oh, that sucks. Ex: A: I failed my exam today. B: Oh man, that sucks. Ex: A: I got laid off from my job today. B: I'm so sorry to hear that. Are you alright?