What is the difference between 'X only if Y' and 'If X, then Y'?

They are both diagrammed the same way, X --> Y.

none.

There is no difference between those two statements that's why it's diagrammed the same way. "only if" introduces a necessary condition and so does "then"

Hmm. I thought so too but apparently, Kaplan doesn't agree with us.

PT 44, Section 2, #25

This was their explanation for the answer...

25. (E) Flaw
Instantly turn “X only if Y” into “If X then Y,” and
watch for arguments that misapply the terms.
The author asserts that, “If technology is accepted,
then it coheres with society’s values.” That’s fine. But
by pointing to the bicycle’s disappearance after an
initial embrace as indicative of a change in values, she
is making the classic error of negating the terms; we
cannot assume that if technology is rejected (as the
bicycle was), then it must not cohere with society’s
values. The author ignores any number of reasons—
other than value-related ones—that the bicycle could
have disappeared for 30–40 years, among them the
unreliable workmanship of the first models, the
prohibitive cost of manufacturing and hence of
purchasing them, laws passed against them, and the
popularity of alternative means of locomotion. (E)
cogently points out this flaw.

kaplan sucks

nothing in that explanation suggests that those two are any different.

in fact it tells you to switch one form to another as soon as you see it.

Yep, nevermind. I read it fast and thought it said it "turns" instead of "turn." Wow, what one 's' can do...

Basically it's the exact same.

Sometimes though, I've found some of the games to be tricky with these conditions.

X only if Y.

Clearly X cannot show up, ever, unless Y shows up first. Of course Y can show up without X.


If X then Y.

Here, X can show up before Y, but then Y must pops up. Of course Y can pop up no matter what.

There might be a question on a game, i can't think of one off the top of my head, but probably something like if Z goes to the party who else can go, a question like that might change the answer a little bit. Or when you're tallking about order, the two have different orders for the first Y must go before X for the second, I don't believe it matters.

In general, it doesn't make much of a difference. Games are getting easier by the day and so they usually don't have these tricky little things.