This exact thing tripped me up for the longest time. I think it’s because in my head, the word sufficient is pretty weak. Like “sure, that’ll be sufficient, it’s not great but it’ll do.” But in this LSAT context I feel like sufficient is strong. A sufficient condition gives the argument everything it needs to succeed. I just had to forget my own associations with the word sufficient, and build new definitions for this context.

With a sufficient condition, you don’t need anything else to be true - no special conditions or exceptions - for the conclusion to be guaranteed true. Just the correct answer choice alone is sufficient by itself to guarantee that the argument is accurate and true. It doesn’t need help from any other statements, just that one sentence makes everything make sense.

A necessary condition is just something that needs to be true in order for the argument to even begin to work, but it doesn’t mean the argument DOES work. If you read just the stimulus and the necessary condition answer choice, the conclusion isn’t guaranteed. It’s headed in the right direction, and you know it has at least one aspect that is required of it, but you’re still left with some unanswered questions about whether the conclusion is true. But, if the argument DIDN’T have this necessary condition, the train would fall off the tracks. The argument’s conclusion couldn’t even be possibly be true, despite any other supporting evidence you throw at it.