Generally, I agree with Michael. "Rue" is an old word. The only phrase I've ever used it in is: "You'll rue the day when..." - and that's something you say more as a joke than anything else. Avoid the word. "Expend" is to use up a finite resource, such as energy. "Spend" is nearly always used with currency, unless you are "spending time" with someone or on something. As for #3, I'd say "I was accepted to the Master's program in Economics at XXX University."

Thank you Michael! It is very helpful!

1) The main difference between "regret" and "rue" is that people today still use the word regret. Pretty much nobody says rue. You might hear "rue" in a movie where the setting is in the 1800s. 2) Expend is to use up something of value. Usually it's something that is in limited supply. "The world's population is expending natural resources faster than ever before." Spend is to use something like money or some other thing of value, usually in exchange for something in return. "I spent all my money on a new car." Spend can also be used with time. "I spent all day in bed." 3) I received an offer of admission to the master's program in Economics at XXX University.