There are some expressions showing your sympathy, such as "That's too bad" or "I'm sorry to hear that." But I'm not sure how to use them correctly.
I've just read a grammar article which says that you never use the sentence "That's too bad" or "Too bad" without sarcasm. Is that true? There are two examples. One is when someone had to work for two weeks without any days off. Another is when someone's mother has been hospitalized because of her illness. In these situations, what sentence is most appropriate to use showing my real sympathy? I thought I could say "That's too bad."
Can you tell me what sentence is good for these situations? And if there is any difference between these two sentences, can you explain it?