Hi @paige4942, I'm assuming you're referring to the about 29:30 into the Intro to Logical Reasoning Video with Mehran. In the example he gives (#2), he provides evidence from a Johnny Cash song where a man is named Sue. Given this information, we know with certainty that one of the premises is false. This is one way to attack an argument—you can attack premises by claiming that they are false. This is likely the most common form of argumentation that you encounter in your day-to-day life.
On the LSAT, you would identify a false premise as a stated assumption that does not hold true given the facts you know of the world. Luckily for us, this is essentially a non-issue on the test, and as Mehran points out in the video shortly after explaining #2, for almost every LSAT question, what we're trying to do is see where the gap is between the premises and the conclusion. Generally, on the LSAT we assume that the premises provided are true and then look for holes in the argument as to why the stated premises do not necessarily support the conclusion, or why the conclusion doesn't necessarily follow from the stated premises.
Questions in which one or more assumptions in the stimulus are attacked (that is, considered a false premise/premises) are very rare, and these types of questions are usually found within the Argument Exchange, Weaken, or Errors in Reasoning section.
Basically, assume that premises on the LSAT are true, and focus on understanding the argument and where the gap(s) are between the premise(s) and the conclusion.
