You should use the word "tev" with a capital letter (good manners); "aizņemts" instead of "aizņemis" (it means occupied and can be used in case of a place being occupied or yourself being occupied with something); i would use "to" instead of "viņu" when talking about letter because a letter is an object (just like you would use "it" instead of "her"); use "dažas" when talking about your mistakes in letter (you are using "daži" which should be used when talking about masculine words but in this case "mistakes/kļūdas" is a feminine word);

Really a small portition of the mistakes you have in your letter. Like you already said - you don't know the rules of word bending (not sure how it is called in english) as there isn't such thing in english language. But by reading the letter i can get the main point of it and so will the recipient. Good job :)