When you use the verb [shimasu] to describe an action - like reading a book or playing a sport - you use the particle [o] to mark the object receiving the action. follows the sport being played, the book being read, and so on.
|I play tennis.||
||Tenisu o shimasu.|
|Kenta played tennis.||
||Kenta wa tenisu o shimashita.|
|What do you play?||nani o shimasuka|
|What did you play?||nani o shimashitaka|
|I will play tennis tomorrow.||
ashita tenisu o shimasu.
|I will play tennis today.||
kyoo tenisu o shimasu.
|I played tennis today.||
kyoo tenisu o shimashita.
|I played tennis yesterday.||
kinoo tenisu o shimashita.