I assigned a letter to my student that invites his friend to a dinner party at his home. He was expreceted to write the letter in informal style.
His letter is:
Haven’t seen you for a long time, how are you? I want to share with a happy thing——I moved to my new home! Our whole family is very excited, so we decide to hold a dinner party Thursday October 4 at 7pm at my new home. If you are free, just give me a call before the dinner.
I hope to see you soon.
I made some changes. Could you proofread it and make it informal? Thank you!
Haven’t seen you for a long time, how are you? I want to share good news with you – I move to my new home! I am very excited (the previous subject is ‘I’), and (I think ‘and’ is more natural in terms of logic. What is your opnion?) will throw a dinner party at 7 on Thursday, Oct. 4 at my new home. If you can come, just give me a call.
Hope to see you soon.
You need the "how are you?" to be its own sentence.
Your change to "I want to share good news with you" is better, but "some good news" is better yet.
There is no reason the whole family can't be excited - leave that alone.
"So" is a very logical connector. You should leave that alone too.
The order of "on October 4 at 7 pm" was also fine. Don't change your students words when they are not wrong.
There is nothing wrong with "I hope to see you soon."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.