Situation 3
Your aunt has sent you an email to invite you to her birthday party of next Friday. But there is a problem! Next Friday you are going away for the weekend with some friends. Write an email back to her to tell her you can't come.
Say sorry and propose to come another day.
Situation 4
Your friend invited you to his / her party by a text message.
Write a text message back to say that you really want to come and you think it is a great idea!