おもしろい

did you translate 'omoshiroi' into 'interesting'? 

in English <em>fun, funny</em> and <em>interesting</em> have different meanings. 

fun - makes you laugh or smile

- is enjoyable

- happy

eg. playing frisbee in the park was fun

funny - makes you laugh or smile

eg. my brother is funny 

interesting - more serious 

- something to think about/using your brain

- might be something sad or happy

eg. the TV show about the war was interesting

so maybe you thought tennis was interesting, but you might be able to find a better word...

Playing tennis is so interesting.

Yesterday I played tennis for the first time with my son and our neighbors whose son is my son's classmate yesterday <em>(this is too far from it's verb 'played')</em>. we could enjoyed it and I thought felt it was a very interesting sports. I also thought it was an easier sports to play than I had expected, because we played it with a tender soft ball. We played it for two hours. Today, I have a muscleular pains in the whole body, but I will be able to continue playing it. I payed $14 for the court fee, and I don't know whether it is expensive or not. My son joined a tennis club in his school so I also started it to play with him.

Playing tennis is so interesting.

I played tennis for the first time with my son and our neighbors (whose son is my son's classmate) yesterday. We could enjoyed it and I thought it was a very interesting sports. I also thought it was an easier sports to play than I expected - because we played it with a soft tender ball. We played it for two hours. Today, I have a muscular pain in the my whole body is sore, but I will be able to continue playing it. I payed $14 for the court fee. I don't know whether it is that was expensive or not. My son has joined a tennis club in his school so I also started it to play with him.

Good! It sounds like you had a nice time! A few notes:

-All sentences <em>must</em> start with a capital letter

-You <em>must</em> put a space after a period ".", a comma ",", a question mark "?" and an exclamation mark "!"

-I have added in a set of brackets - you <em>don't</em> put a space after the first bracket

-Tennis is <em>singular</em> and is therefore just a <em>sport</em>

-When the next clause of a sentence is going to expand on what has been written in the previous then you should use a colon ":" or a hyphen "-"

Hope this helps!