Due To vs Because Of: The OET Grammar Trap Almost Everyone Gets Wrong

Due to and because of both introduce a reason, and OET candidates reach for both constantly — every referral letter explains why the patient was seen, why a treatment was started, or why a discharge happened when it did. The problem isn’t recognising that a reason is needed. It’s that due to and because of follow different grammar rules, and using the wrong one produces a sentence that reads as confidently wrong rather than simply informal — exactly the kind of error the Language criterion is built to catch.

Watch the full breakdown: Due To vs Because Of works through the swap test using real OET case-note sentences.

Why do due to and because of get confused in OET writing?

Both phrases translate to a single flexible “because” word or connector in many candidates’ first languages, so English’s split between a noun-modifying form and a verb-modifying form doesn’t map onto any distinction candidates are used to making. Add to that the fact that spoken English has been gradually loosening the rule for decades — you’ll hear native speakers say “the flight was delayed due to weather” constantly, and it sounds completely natural — and it’s easy to conclude the distinction doesn’t matter anymore.

It still matters in OET writing specifically, because the Writing sub-test is marked in a formal, professional register, and the Language criterion scores grammatical range and control against that register’s own rules, not against how the phrase gets used in casual speech. A referral or discharge letter that mixes up due to and because of across several sentences reads as a repeated control gap, even though a reader can still follow the meaning every time.

The core rule: what due to and because of are each allowed to modify

The traditional grammar rule treats due to and because of as different parts of speech, which is the whole reason they behave differently.

• Due to functions as an adjective phrase. Adjectives describe nouns, so due to can only attach to a noun — almost always the subject of the sentence, usually right after a form of “to be” (was, is, were, has been).
• Because of functions as a prepositional phrase attached to the reason itself, and it modifies verbs — it explains why an action took place, wherever that action sits in the sentence.

That single distinction predicts almost every correct and incorrect use you’ll encounter in an OET letter.

The swap test: how to choose in ten seconds every time

You don’t need to identify parts of speech under exam pressure — you need a test you can run instantly. Replace the phrase with “caused by” and read the sentence back.

• If “caused by” still makes grammatical sense, due to is correct.
• If “caused by” makes the sentence collapse, you need because of (or plain “because” with a full clause) instead.

Example 1: “The infection was due to poor wound care.” → “The infection was caused by poor wound care.” This works, so due to is correct.

Example 2: “She was readmitted due to a chest infection.” → “She was readmitted caused by a chest infection.” This doesn’t work — “readmitted” is a verb with nothing left for “due to” to describe once you strip out “caused by” as a stand-in. The sentence needs because of instead: “She was readmitted because of a chest infection.”

Example 3: “The appointment was cancelled due to staff illness.” → “The appointment was cancelled caused by staff illness.” This also fails the test for the same reason — “cancelled” is the verb, so because of is the correct connector here, even though “due to” is the version many candidates would write instinctively.

Running every reason-clause in your letter through this test before you submit catches the error pattern in under a minute, because the same handful of sentence shapes repeat across almost every OET letter.

Due to in OET letters: the noun-modifying pattern

Reach for due to only when the reason is describing a noun — usually the subject — that sits right before it, typically connected by a form of “to be.”

Notice that in every row, due to follows a noun (“diagnosis,” “readmission,” “delay”) and a form of “to be,” never a verb describing an action the patient or clinician performed.

Because of in OET letters: the verb-modifying pattern

Because of is the safer default whenever the reason is attached to something someone or something did, rather than to a noun sitting after “to be.”

In each case, because of attaches to a verb phrase describing an action (“was referred,” “was changed,” “was brought forward”) — exactly the position due to cannot occupy under the traditional rule.

Worked examples: the same error, corrected

The pattern across the first four rows is identical: a verb (“was admitted,” “failed to heal,” “was rescheduled,” “declined”) sits directly before the reason, so due to has nothing valid to modify and because of is required. The fifth row is the mirror case — because “recovery” is a noun following “was,” due to is the correct choice there and swapping in because of would be the error.

What criterion does this fall under, and why does it matter?

This is a Language-criterion error under the OET writing marking scheme, which sets out six criteria: Purpose, Content, Conciseness & Clarity, Genre & Style, Organisation & Layout, and Language. Language scores grammatical range and control, and a fixed-rule connector used against its own grammar — like due to attached to a verb — is graded as a control error even when the sentence’s meaning is never in doubt. Because due to and because of both appear so often in referral, discharge, and progress letters, a candidate who hasn’t fixed the distinction tends to repeat the same error three or four times across one script, which reads as a pattern rather than a slip.

It also brushes against Genre & Style, since the traditional rule is a marker of formal, edited written English specifically — the register OET letters are meant to imitate — as opposed to the looser spoken usage where due to has spread into verb-modifying positions without penalty.

A quick self-check before you submit

Find every instance of “due to” in your draft and ask one question: is the word immediately before it a noun following a form of “to be” (was, is, were, has been)? If yes, due to is correct. If the word before it is a verb describing an action — admitted, delayed, cancelled, referred, changed — swap in “because of.” Running the swap test (“replace with caused by — does it still work?”) on any sentence you’re unsure about takes seconds and resolves the choice without needing to name the grammar at all.

This pairs well with our broader guide on connectors and cohesion in OET writing, which covers how linking words carry an argument across a whole letter, and with the Language criterion breakdown, which explains how grammatical range and control are weighted at each band — exactly where a due-to slip does its damage even when the sentence is still understandable. Our clauses of contrast guide covers the equivalent grammar trap for although, despite, and however, if reason connectors are one of several grammar areas you’re working through. For a fast first check on a specific letter, our free OET Grammar Checker will flag many due to/because of mismatches automatically.

Why this is worth fixing as a habit

Due to and because of aren’t occasional connectors in OET writing — between them, they appear in nearly every referral, discharge, and progress letter, often more than once, so a candidate who hasn’t fixed this distinction repeats the same error pattern across a single script rather than making an isolated slip. Examiners marking Language read for exactly that kind of repeated pattern, and a letter where “due to” is consistently attached to verbs reads as a genuine grammatical control gap even though every sentence still communicates its intended meaning. Our OET writing correction service checks fixed-rule connector errors like this across a full letter alongside the other five criteria, so a repeated due-to/because-of slip gets caught and corrected before it costs you a band.