What can you assume is the subject of a sentence that gives an order, such as "Buy your tickets here

In English, the subject of a command, order, or suggestion - you, the person being directed - is usually left out of the sentence and is said to be the understood subject:

[You] Step lively there or I'll leave you behind!

Before assembling the swingset, [you] read these instructions carefully.

I don't know if this is what you're looking for, but I hope I cleared things up for you.