Dear Mr./Ms. XXXX,

We would like to invite you to attend a lecture at my school(your school name). The lecture is about how to improve the English study abilities. You could give the attendees some general rules on how to approve theri English study abilities. Follow by that, please give some examples if possible and then there will be a 15 mins QA(Question and Answer) time for students to ask some questions they interested.

After the lecture, we would like to invite you for a dinner party as well. During the party there will be lots of English professors attend and they are expecting to see on the party.

We hope the lelcture can fit in your schedule. Please kindly reply us in case you cannot attend.

Regards,

Your name

大概就是这样吧，如果是书信形式的话。。。