Informal

Hi Jenny,
Thank you for inviting me for a drink. I'm sorry but unfortunately I can’t come because I am go to the cinema with my son. Friday my ex-wife takes care him, so I'll have time for a meeting. What do you think about a drink on Friday?
See you soon,

Neutral

Dear Marco,
I am writing to you because I want to apologize for the delay. I had family problems, but now everything is fine. I am sorry again and I am attaching the documents.
Looking forward to meeting you,
Alex

Formal

Dear Mrs Daley,

I am writing to to you in connection with your phone from you. I listened your message left on voice mail. We have catalogs available to large and small steel nails. Both types should include products that interested in you. You can also subscribe to these directories, it costs 6 pound. Indicated by you Catalog will send within 7 days. If you have any further question please do not me hesitate to contact me.

I look foroward to haering from you soon.

Your sincerely.