So, we went to a beer garden with our friend's to celebrate his birthday. So we went to a beer garden with our friend's to celebrate his birthday.

We had prepared a birthday plate to suprprise him without telling him. We had prepared a birthday plate to surprise him without telling him.

English = perfect. But a birthday plate is not an expression we use. Do you mean cake? present? If you truly mean plate then you would need to write; "We had prepared a special plate for his birthday to surprise him without telling him." Also Surprise means not telling him --> so --> "We had prepared a special plate for his birthday to surprise him" -- or -- ""We had prepared a special plate for his birthday without telling him"

It looked like he was verry happy when my other friend brought the plate, but at the same time, I was also suprprised because my name was also written on the plate like " Happy Birthday Shinpei(my friend's name) and Kazu(my name)". It looked like he was very happy when my other friend brought the plate, but at the same time, I was also surprised because my name was also written on the plate like " Happy Birthday Shinpei(my friend's name) and Kazu(my name)".

Actually, it's my birthday on June 29th, so my they had also prepared it for me without me knowing. Actually, it's my birthday on June 29th, so they had also prepared it for me without me knowing.