All over the world children (and teachers) are going back to school after the summer break, and this cartoon by Rick McKee from The Augusta Chronicle is one of a whole 'Back to School' collection on The Cagle Post.
An old-fashioned teacher holding a ruler, is giving a pupil the time-honoured back-to-school writing assignment: 'What I did this summer'. The boy, who is playing with his mobile phone, tells her, "If you followed me on Twitter, you'd know." See here for a very similar cartoon from 2013.
The cartoon features a nice example of the second conditional, which is used to talk about ‘unreal’ or impossible things. Note that 'you'd' is a contraction of 'you would'.
• If I won a lot of money I’d buy a big house in the country.
• Where would you live if you could live anywhere in the world?
• If you didn’t smoke so much you’d feel a lot better.
The structure is usually if + past simple and would + infinitive. It’s not important which clause comes first.