One of the most basic concepts on the LSAT is the idea of conditional statements. The simplest form is a straightforward if-then statement. In a contract, it might look like this: "I promise that if I am approved for the loan, then I will buy your house."
This promise means that if you get the loan, we have sufficient information to know that you are contractually obligated to buy the house. And if you don't get the loan, then we have sufficient information to know that you're not obligated to buy the house. This is super-important: The promise only applies IF you are approved for the loan. If you're NOT approved for the loan, the promise has no effect whatsoever--it's as if the promise doesn't even exist. Even if you're turned down for the loan, you could still buy the house. Maybe you win the lottery, maybe you find $500,000 under your mattress, whatever--your promise to buy the house if you DO get the loan does not prevent you from buying the house through some other means if you DON'T get the loan.
Here's another simple example:
"If you're Warren Buffett, then you're rich."
This statement means that if you ARE Warren Buffett, then we have sufficient information to know that you are rich. (Buffet-->Rich). What the statement doesn't mean is that anyone who is rich is Warren Buffett. There are plenty of other rich folks in the world. But the statement does mean that if you're NOT rich, then you are NOT Warren Buffet. (NOT Rich-->NOT Buffet).
What I just did there is called the "contrapositive." Memorize this: To do the contrapositive, you must 1) switch the order of the terms, and 2) negate the terms. It's very puzzling to new students, but pretty straightforward once you get used to it.
"If you trip and fall headfirst into an industrial sausage grinder, then you are dead."
Okay, so if someone gets turned into sausage, that's sufficient information for us to know that they are dead. (Sausage-->Dead). But that doesn't mean that every dead person fell headfirst into an industrial sausage grinder. The arrow only goes one way. What it does mean is that if you are NOT dead, then you could NOT have fallen into an industrial sausage grinder. When we draw the contrapositive, we'll see the terms in 1) opposite order with 2) the opposite sign. So the contrapositive is (NOT dead-->NOT sausage). If someone is NOT dead, then we have sufficient information to know that they are not sausage.
Also memorize this: The sufficient condition is always on the left. Next time, I'll revisit this same concept and introduce the term "necessary."