Ensure, Insure, or Assure? The OET Vocabulary Trap

“Please ensure the patient that her results are normal” looks almost right. It reads smoothly, every word is spelled correctly, and a quick skim won’t flag anything. But the sentence is wrong — the writer meant “assure,” not “ensure,” and the letter now asks the reader to make certain a person, which isn’t something you can do to a human being. This is the kind of error OET vocabulary confusions specialise in: three real, correctly spelled English words — ensure, insure, and assure — that sound almost identical in connected speech but carry three different meanings, each with its own grammatical pattern.

Why do ensure, insure, and assure keep getting confused in OET letters?

All three words trace back to the same Latin root meaning “to make secure,” which is exactly why they sound so alike and why English speakers — native and non-native alike — reach for the wrong one under time pressure. In connected speech, “ensure” and “insure” are near-homophones; many speakers barely distinguish the first vowel sound at all. “Assure” shares the same final syllable and the same general sense of certainty, which is enough to make all three feel interchangeable in the moment you’re writing a referral letter in 35 minutes.

The problem is compounded for OET candidates specifically because none of the three words is a spelling mistake. A spellchecker sees “ensure,” “insure,” and “assure” and confirms all three are valid English words — it has no way to know which one belongs in your sentence, because that depends entirely on meaning and on what kind of object follows the verb. This is a Language-criterion error that survives every automated check, and it only shows up when a trained reader checks the sentence against what it’s actually trying to say.

What does ensure mean, and when do you use it?

Ensure means to make certain that something happens, is done, or is the case. It is the most frequently needed of the three words in clinical writing, because so much of a referral, discharge, or advice letter is about making sure actions are carried out correctly.

• ✅ “Please ensure the dressing is changed every 48 hours.”
• ✅ “Ensure she continues the course of antibiotics for the full seven days.”
• ✅ “I would ensure that follow-up bloods are taken in two weeks.”

Ensure always takes a fact, action, or state as its object — never a person. You ensure that something happens; you don’t ensure a person.

What does insure mean, and when do you use it?

Insure means to arrange for financial compensation in the event of loss, damage, illness, or death, typically through an insurance policy. It belongs almost exclusively to financial and insurance contexts, which is why it appears so rarely in genuine clinical writing — and why most OET candidates who write “insure” actually meant “ensure.”

• ✅ “The practice insures its diagnostic equipment against accidental damage.”
• ✅ “He was advised to insure against loss of income during his extended recovery.”
• ❌ “Please insure the medication is taken as prescribed.” (should be ensure)

If your sentence doesn’t involve a policy, a premium, or compensation for a financial loss, “insure” is almost certainly the wrong word, however natural it feels in the moment.

What does assure mean, and when do you use it?

Assure means to tell a specific person that something is true in order to remove their doubt or worry. Unlike ensure and insure, assure takes a person as its direct object — you assure someone of something, not a fact by itself.

• ✅ “I assured Mrs Patel that the scar would fade with time.”
• ✅ “The consultant assured him that the procedure carries minimal risk.”
• ✅ “Please be assured that her vital signs remained stable throughout.”

A useful test: if you can finish the sentence with “…that [a person] won’t worry anymore,” you need assure. If the sentence is about guaranteeing an outcome rather than reassuring a person, you need ensure.

Ensure, insure, and assure side by side

Does mixing these up actually cost Language marks?

Yes, under the same occasional-versus-systematic distinction examiners apply to every Language-criterion slip. A single “insure” written where “ensure” was meant, in an otherwise accurate 180-200 word letter, may not move the needle on its own — examiners are instructed to tolerate occasional errors that don’t block communication. The risk with ensure/insure/assure is the same as with any fixed-rule word pair: candidates who haven’t learned the rule don’t make the mistake once, they default to the same wrong word every time the sense is needed, and a single slip becomes a repeated pattern across the letter.

There’s also a Content risk specific to assure. Writing “ensure the patient” where you meant “assure the patient” doesn’t just read oddly — it asks the reader to do something that doesn’t make grammatical sense, which can momentarily obscure what action you actually want taken. In a referral or discharge letter, where the reader is often working quickly, that moment of confusion is exactly the kind of friction the Clarity criterion is designed to penalise.

A self-check for ensure, insure, and assure before you submit

Because none of these three words will ever be flagged by spellcheck, they need a targeted search-and-check pass rather than a general read-through.

1. Search your letter for every instance of “ensure,” “insure,” and “assure.” Isolate each one before judging it.
2. For each instance, identify the object. Is it a person (assure), a fact or action (ensure), or a financial/insurance arrangement (insure)?
3. For “insure” specifically, ask whether a policy or compensation is actually involved. If not, change it to “ensure.”
4. For “assure,” confirm a specific person is named or implied as the one being reassured. If the object is an action rather than a person, you need “ensure” instead.
5. Read the sentence aloud with the other two words substituted in. If more than one substitution still makes grammatical sense, you haven’t yet confirmed which one is correct — check the case notes again for what you’re actually trying to say.

This check takes under a minute once you know to search for all three words specifically, and it catches an error type that passes every automated grammar tool because every word involved is genuinely correct English.

How this connects to the OET marking criteria

Ensure/insure/assure errors are marked under Language, one of the six official OET writing criteria alongside Purpose, Content, Conciseness & Clarity, Genre & Style, and Organisation & Layout. For the broader set of grammar and vocabulary patterns that most often cost Language marks, see our OET writing grammar guide, and for a closely related case of real, correctly spelled words competing for the same sentence, see word confusions in OET writing: affect/effect, prescribe/proscribe. The official Language criterion page sets out how this criterion is scored band by band. For a worked video walkthrough of this exact trap, watch the full breakdown.

Ensure/insure/assure is also a useful reminder that vocabulary range doesn’t protect your Language score on its own. A candidate who writes fluent, varied clinical English is just as exposed to this error as a candidate working at a more basic level, because the fix isn’t a bigger vocabulary — it’s learning three specific words as three specific rules, rather than trusting that the right one will simply feel right in the moment.

The honest limit of catching these yourself

Ensure, insure, and assure are difficult to self-edit precisely because all three are familiar, correctly spelled English words — your eye has no reason to pause on any of them unless you already know to check. The free OET Grammar Checker is a reasonable first pass for surface-level errors, but a confidently wrong word choice that reads smoothly and passes spellcheck is exactly the kind of mistake that benefits from a second, trained reader working against your actual case notes. Our OET writing correction service has corrected 11,000+ letters since 2014 and reviews your letter against all six official criteria — including the word-level precision errors that never look wrong to the person who wrote them. Compare options on the pricing page.