Question

2. Identify the subject of a sentence when the subject is not stated. (TIP: get rid of prep phrase)
Before assembling the swingset, read these instructions carefully.

Answer 1

Is the sentence talking to a swingset?
Let's pretend that someone said this to you, who's supposed to read the instructions?

Answer 2

me

Answer 3

"you"

Answer 4

Right. It's understood "you".

Answer 5

so its you the subject

Answer 6

Yep. In a sentence like this, the "you" is often called, "Understood you."