Corrections are red

Suggestions are blue

Green is everything else

Dear Mike

Dear Mike,

First at all I would like to thank you to do for making this course so entertaining. As I’ve said to my school lead staff today, it’s very difficult to spend all the whole morning teaching at to the same group without it becoming boring, and you’ve got it doing the course entertaining at the same time that you’ve taught us a lot of this fascinating language as English is.

I think you wanted to say: "and you've made it entertaining while teaching us a lot of interesting things about English", but it is hard to understand that part of what you wrote. 
 
Now I want to prepare for the September text level, but I need some recommendations, as I’m not sure if it is better to prepare it with the book that we’ve been using this summer or is it better to use my daughter Jane's forth-level book? (Or maybe is better a new book that you recommend to me.)

On the other hand, I also want request ask you if I must I work on both grammar and vocabulary, or basically just grammar?

"On the other hand" is used when you say something that goes against what you said before. You don't use it to change the subject, or to add something further. For example, "I love your first book. On the other hand, I love your second book too" is wrong. "I love your first book. On the other hand, I don't like your second book" is right.

I would be very grateful if you can help me, but if it is not possible, don’t worry I understand that you must have a lot of work.

Kind regards.

PS: Sorry about the mistakes, but I’ve thought that it was better to write with some mistakes than don’t writing not write in English.

You did a good job!  It is always better to try writing in the language you are learning. Remember to always include the object in sentences. Unlike Spanish, you almost always have to have an object in English.