- Your mobile phone battery is almost empty.
你的手机快没电了。
For the below sentences, I assume you want the imperative form, and not the root form of verbs.
- Play a game on your mobile phone.
在手机上玩(一个)游戏。
- Play a game on your mobile phone outside the classroom.
在教室外玩手机游戏。
- Take a photo of a plant through a window.
透过窗口给植物拍照
- Play a game on phone in a place that has lots of sick people.
在有很多病人的地方玩手机游戏。
Hope this helps, Ryokan :)