Level 5 — Advanced · Technical & Evidence-Based Writing
Task 67: Writing a Conditional Rule
Write a rule using if, only if, and if and only if for exactly the logical direction you mean — the system Lesson 67 taught you to read.
Learning Objectives
By the end of this task, you should be able to:
- Use 'if' for a sufficient condition and 'only if' for a necessary one, correctly
- Use 'if and only if' only when both directions genuinely hold
- State an 'unless' exception that carves out a real excluded case
Introduction
Lesson 67 taught you that "if," "only if," and "if and only if" claim three different logical directions, not the same thing in different words. This task asks you to write a short rule for each, correctly.
The Rule, for Writing
Core Concept
"If A, then B" claims A is enough to guarantee B (sufficient). "B only if A" claims A is required for B (necessary), without promising A guarantees B. "B if and only if A" claims both directions — A and B always occur together. Use "unless" to carve out a genuine exception to a stated rule.
Example
If a payment is received before the deadline, the order ships the same day (sufficient). The order can only be shipped if the address has been verified (necessary, not sufficient). A refund is issued if and only if the item is returned within 14 days in its original condition (both directions).
Your Turn
Your Turn
Write three short rules for a real or realistic system or policy: one using "if... then" for a sufficient condition, one using "only if" for a necessary condition, and one using "if and only if" for a claim where both directions genuinely hold. Add one "unless" exception to any of them.
Vocabulary in Context
sufficientadjectiveformal
enough by itself to guarantee an outcome (পর্যাপ্ত)
Often with sufficient condition, sufficient evidence, not sufficient
“A signed release is sufficient for the gate to open the container.”
necessaryadjectiveformal
required for an outcome, though not always enough by itself (আবশ্যক)
Often with necessary condition, strictly necessary, necessary but not sufficient
“A valid licence is necessary but not sufficient to operate a crane.”
Model Answer
Show a model answer
If a driver completes the safety induction, they are cleared to enter the yard (sufficient — the induction alone guarantees clearance). A driver can only enter the yard if they are wearing a high-visibility vest (necessary — the vest is required, though it doesn't by itself guarantee entry, since the induction is still needed too). A shipment is marked complete if and only if both the delivery is confirmed and the paperwork is signed (both directions must hold together), unless the customer has pre-authorised delivery without a signature.
Self-Check
Self-Check
- Did your "if" rule genuinely claim sufficiency, not just correlation?
- Did your "only if" rule state a requirement without overclaiming it guarantees the outcome?
- Did your "if and only if" rule genuinely hold in both directions?
- Did you write directly in English, without composing the passage in Bangla first?
Next: Writing 68 — Writing a Rule with a Genuine Exception: telling an exception apart from a limitation or a precaution, on the page.