Your sister told you that she's going to have a baby. You're happy and you want to congratulate her. You say:
Oh my God, that's wonderful!
This is an expression that people use when they're shocked or surprised. This can be for positive reasons, like when someone says that they're pregnant, or for negative reasons like when someone has gotten in an accident.
When someone tells you good news, you might expect to say "congratulations". This would be the polite way to react if a coworker or not-very-close friend said that she was going to have a baby. But with someone really close like your sister, "Congratulations" is too distant. So a more excited response like "Oh my god!" is better.
Use "that's" instead of "it's" to respond to something that a person tells you.
"Wonderful" means "great" or "very good". But it has a more extreme and more positive meaning. When you say that something is "wonderful", it means that it makes you really happy. Some examples include:
A: How was your trip?
B: It was wonderful. We had such a great time.
A: I got the job!
B: Oh, that's wonderful!
"Wonderful" is an ungradable adjective, which means that it sounds strange to use words like "very" or "a little" with it. That's because "very" is already part of the meaning of the word "wonderful". Some other ungradable adjectives include:
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