Subject
Note that if the subject of a sentence is clear from the context it can often be dropped. In the following the red sentences are without a subject.
王李：张红，昨天的足球比赛怎么样？
张红：很有意思。今天天气很好，我们去打网球，好吗？
王李：太好了！ 什么时候去？
张红：现在去，可以吗？
王李：可以。
The implied subject of 很有意思 is 足球比赛.
The implied subject of 什么时候去 is 我们, i.e. Wang Li and Zhang Hong.
The same is true for 现在去。
In the case of 可以吗 and 可以, you would need to add 'it' in English.