Key takeaways
- "A only if B" means: if A, then B. The word after "only if" is what's required.
- "A unless B" means: if not B, then A. Cross out "unless," write "if not."
- Every if-then statement has one valid flip, the contrapositive: if not B, then not A.
- The most tested mistake is the reversal: reading "if A, then B" as "if B, then A."
Conditional logic shows up across the LSAT, in Must Be True, Sufficient Assumption, Flaw, Parallel and Principle questions. Most of the mistakes I see in students' work come down to two words: "unless" and "only if." Both look simple. Both get translated backwards under time pressure. This guide gives you a fixed rule for each, shows you how to form the contrapositive, and walks through an original question.
The building block: if-then
A conditional statement says one thing guarantees another. "If you are a member, you get a discount" means: being a member is enough to get the discount. On the LSAT we write it as:
Member → Discount
The left side is the sufficient condition (enough on its own). The right side is the necessary condition (required). Every rule below turns a tricky word into this one shape.
Rule 1: "only if" introduces the requirement
A only if B → If A, then B (A → B)
"You can enter only if you have a ticket" means having a ticket is required. So: Enter → Ticket. It does not mean having a ticket gets you in. Maybe you also need ID.
The trap is that "only if" sounds like "if," so students write B → A. Fix it with one habit: the idea right after "only if" goes on the right side of the arrow. Always.
Watch for the close cousins:
| Wording | Translation |
|---|---|
| A only if B | A → B |
| Only if B, A | A → B |
| A requires B | A → B |
| B is necessary for A | A → B |
| A if B | B → A |
| Only A are B | B → A |
| A if and only if B | A → B and B → A |
"Only" by itself (as in "only members may vote") works like "only if": the group after "only" is the requirement. Vote → Member.
Rule 2: "unless" means "if not"
A unless B → If not B, then A (not B → A)
Here's the move I teach: cross out "unless," write "if not," and keep everything else where it is.
"The picnic will be canceled unless it is sunny" becomes "The picnic will be canceled if not sunny":
Not sunny → Canceled
That's the whole rule. It works no matter where "unless" sits in the sentence. "Unless it is sunny, the picnic will be canceled" is the same statement.
"Except if," "without" and "until" usually work the same way. "Without a permit, you can't build" means Not permit → Not build.
A note on "no… unless." "No one can vote unless they are registered" becomes "No one can vote if not registered": Not registered → Not vote. Its contrapositive is Vote → Registered, which is how most people think of it anyway.
The contrapositive: the one valid flip
Every conditional has exactly one other form that means the same thing. Flip the two sides and negate both:
A → B is the same as Not B → Not A
- Enter → Ticket becomes No ticket → Not enter.
- Not sunny → Canceled becomes Not canceled → Sunny.
Check that second one in plain words: if the picnic wasn't canceled, it must have been sunny. Correct.
Two moves look like the contrapositive but are invalid:
- Reversal: A → B, so B → A. ("You have a ticket, so you can enter." Not proven.)
- Negation: A → B, so Not A → Not B. ("You didn't enter, so you have no ticket." Not proven.)
The LSAT builds wrong answers, and many Flaw questions, out of exactly these two moves.
Chains
Conditionals link when the right side of one matches the left side of another:
A → B and B → C give you A → C, and its contrapositive Not C → Not A.
When you have three or four statements, translate each one, write the contrapositives, then look for matching terms to link. Most Must Be True answers built on conditional logic are just the end of a chain or its contrapositive. See Must Be True and Most Strongly Supported questions for how those questions use this.
Sample question (LSAT JET original)
No employee at Ridgeline Labs can access the archive unless they have completed safety training. Every employee who has completed safety training has been assigned a mentor. Priya, an employee at Ridgeline Labs, has not been assigned a mentor.
If the statements above are true, which one of the following must also be true?
(A) Priya has completed safety training. (B) Priya cannot access the archive. (C) Every employee who has been assigned a mentor can access the archive. (D) Priya will be assigned a mentor once she completes safety training. (E) Some employees who can access the archive have not been assigned a mentor.
Answer: (B).
Translate:
- "No employee can access the archive unless trained" → Not trained → Not access. Contrapositive: Access → Trained.
- Trained → Mentor. Contrapositive: Not mentor → Not trained.
Chain: Access → Trained → Mentor, so Not mentor → Not trained → Not access.
Priya has no mentor, so she isn't trained, so she can't access the archive. That's (B).
- (A) contradicts the chain: no mentor means not trained.
- (C) reverses Access → Mentor. Not proven.
- (D) is plausible but goes beyond the rules. "Trained → Mentor" says trained employees have mentors; it says nothing about timing.
- (E) contradicts the chain: everyone with access has a mentor.
How to practice translation
- Take ten conditional sentences from your practice tests.
- Translate each into A → B, then write its contrapositive.
- Say each one out loud in plain English to check it makes sense.
- Time yourself. Aim for each translation in under 15 seconds before moving to full questions.
Translation is a skill you drill, not a fact you memorize. Our Conditional Logic Lab, part of the LSAT JET student platform, is built around exactly this kind of repetition.
Frequently asked questions
How do you translate "unless" on the LSAT?
Replace "unless" with "if not" and keep everything else in place. "A unless B" becomes "If not B, then A." The contrapositive is "If not A, then B."
What does "only if" mean on the LSAT?
"A only if B" means "If A, then B." The idea after "only if" is the necessary condition, so it always goes on the right side of the arrow.
What is the difference between "if" and "only if"?
"A if B" means B → A, so B is sufficient. "A only if B" means A → B, so B is necessary. The same two letters point in opposite directions.
What is a contrapositive on the LSAT?
It's the one valid flip of a conditional: reverse the two sides and negate both. "A → B" becomes "Not B → Not A," and the two mean exactly the same thing.
Next step
Small live classes, capped at 12 students. Alan teaches every session himself and reviews your mistakes.
Conditional logic is session 2 of our LSAT Foundations track. Book a free 15-minute course call to see if the January cohort fits your schedule.
Related articles
- LSAT Sufficient Assumption Questions: Build the Bridge
A sufficient assumption makes the conclusion airtight. Find the new idea in the conclusion and connect it to the evidence.
- LSAT Must Be True and Most Strongly Supported Questions
The right answer is already in the passage. Your job is to prove it, word by word, and to distrust anything new.