In 1799, a preserved platypus specimen arrived at the British Museum — and the scientists examining it could not agree on what they were looking at. It must be a fake, some insisted. Someone couldn’t have created this naturally. Others thought it might be a real animal, though how? A mammal that lays eggs, has a duck’s bill, and uses electroreception to hunt underwater? It can’t be real.
But it was. Completely, stubbornly real. And those confused European scientists were doing something enormously useful: using logic and available evidence to draw conclusions about the unknown. That is precisely what modals of deduction allow you to do in English.
Today we are going to master three of them — must, might/could/may, and can’t — using Australia as our context. A continent of ancient civilisations, extraordinary wildlife, and a few natural wonders that defy easy explanation. Perfect territory for making logical deductions.
The Rule: Signalling How Certain You Are
| Certainty | Affirmative modal | Negative modal | What it means |
|---|---|---|---|
| ✅ Almost certain | must |
can't / couldn't |
“I am highly confident this is / is not the case.” |
| 🔄 Possible | might / could / may |
might not / may not |
“This is one possibility — I am not certain.” |
Formation: present, continuous and past
| Time reference | Structure | Example from Australia |
|---|---|---|
| Present / general | modal + bare infinitive |
That call must be a kookaburra. |
| Present continuous action | modal + be + verb-ing |
She must be studying in Brisbane. |
| Past (completed) | modal + have + past participle |
Those settlers can’t have been prepared for this landscape. |
| Past (ongoing action) | modal + have been + verb-ing |
They must have been walking for weeks to cross the desert. |
“You must carry water in the outback.” (obligation — you need to do this)
“You’ve been walking for three hours — you must be exhausted.” (deduction — I conclude this from evidence)
Context always makes the meaning clear.
🌏 Australia Through the Lens of Deduction
Australia is a continent that rewards careful, logical thinking. Its wildlife is unlike anywhere else on Earth, its landscapes are extreme and ancient, and its human history spans at least 65,000 years. Here are six verified facts — each one ideal for practising modals of deduction.
When the first preserved platypus reached Europe, British naturalist George Shaw suspected it must be a taxidermist’s hoax — stitched together from different animals. It was not. The platypus lays eggs, yet nurses its young with milk; it has a duck’s bill that detects electrical fields underwater. It must have evolved along a path completely separate from other mammals.
Archaeological evidence places Aboriginal Australians on the continent for at least 65,000 years — the world’s oldest continuous civilisation. To have sustained rich cultures across such a vast and often harsh landscape for so long, those early communities must have built extraordinarily detailed knowledge of the land, its water sources, and its seasonal rhythms.
Australia’s human population is around 27 million. The kangaroo population? Roughly 50 million. There must be more kangaroos than people — and indeed there are. In some outback regions, kangaroos outnumber humans by dozens to one. That silhouette at dusk on the roadside might be a car — or it might be a two-metre red kangaroo.
Stretching over 2,300 km along Australia’s northeastern coast, the Great Barrier Reef is the largest coral reef system on Earth — visible from space. Something that vast can’t have formed quickly. Scientists estimate the current reef structure has been growing for 6,000 to 8,000 years, though some underlying formations are far older.
Australia is home to ten of the world’s fifteen most venomous snake species. If you encounter a snake in the Australian bush, it might be a harmless green tree snake — or it could be an eastern brown, responsible for more snakebite fatalities than any other Australian species. When in doubt, treat it as though it must be dangerous.
Around 80% of Australia’s mammals, reptiles and frogs are found nowhere else on Earth. Australia separated from Antarctica roughly 40 million years ago, and its long geological isolation must have allowed evolution to take a radically different path. Any unfamiliar animal you spot in the bush might be found on no other continent.
⚠️ Common Mistakes for Spanish Speakers
Five errors to watch — and how to fix them
✅ “That kangaroo must be thirsty.”
Never add to after a modal verb. Spanish speakers often translate “debe ser” as “must to be”, but English modals always take the bare infinitive — no “to” ever follows must, might, can’t, could, or may.
✅ “She can’t be at home — the lights are all off.”
This is the most frequent error Spanish speakers make. Mustn’t signals prohibition (“You mustn’t feed the kangaroos”), not negative deduction. For a strong negative deduction, always use can’t or couldn’t.
✅ “He must be exhausted after that hike.”
Should expresses expectation or advice, not confident logical deduction. When you are drawing a conclusion from clear evidence — a 30 km trek in 40°C heat — use must to signal that confidence.
✅ “That platypus could be real — look at the evidence.” / “That platypus must be real — the evidence is undeniable.”
Affirmative can is not used for deduction about a specific situation. For weak deduction, use could or might; for strong deduction, use must. (Note: “can” is fine for general possibility — “It can snow in the Snowy Mountains in July” — just not for reasoning about a particular case.)
✅ “Aboriginal Australians must have arrived by sea 65,000 years ago.”
For a completed past action (the arrival happened and ended), use must have + past participle. Save must have been + -ing for when you want to emphasise the ongoing nature of a past action: “They must have been travelling for weeks to cross the continent.”
📋 Example Bank: 18 Sentences from Australia
Each sentence models how native speakers use deduction in natural conversation. Notice the variety of tenses.
- That animal has a duck bill, webbed feet and a flat tail — it must be a platypus.
- The Aboriginal rock paintings in Kakadu are over 30,000 years old. The artists can’t have used modern pigments.
- The kangaroo keeps looking towards the river. It must be very thirsty in this heat.
- I found a large, funnel-shaped web near the tent entrance. There might be a funnel-web spider nearby.
- The Great Barrier Reef stretches over 2,300 kilometres. It must be the largest living structure on Earth.
- She has lived in Sydney for twelve years, but she still might not know every suburb — it is a vast city.
- The sun does not set until 8 pm in Darwin. Darwin must be further north than I imagined.
- Nobody is at the beach and the shops are all open. It can’t be a public holiday today.
- The flight from Madrid to Sydney takes over twenty hours. Australia must be extraordinarily far from Europe.
- Those dark clouds are moving in fast over Uluru. It might rain before we reach the viewpoint.
- He has worked as a park ranger in the Blue Mountains for seven years. He must know every trail by now.
- The temperature reached 46°C in Alice Springs last week. It can’t have been comfortable to work outdoors.
- There are fresh tyre tracks in the red sand. Someone must have driven through here this morning.
- In 1799, those European scientists couldn’t have imagined that the platypus specimen was genuine.
- Eighty per cent of Australia’s mammals are endemic. They must have been evolving in total isolation for tens of millions of years.
- The coral structures in the reef couldn’t have formed in just a few centuries — ecosystems like this take millennia to develop.
- The Anangu people have passed on their knowledge of country through story for 65,000 years. Those stories must contain detailed ecological wisdom we are only beginning to understand.
- If early Australians navigated the entire continent before recorded history, they must have had remarkably sophisticated systems of communication and navigation.
🧩 Quick Practice Quiz
Five questions — one correct answer each
1. You see a large, dangerous-looking spider on the hostel wall in Queensland. Your friend says: “It _____ be harmless — it is far too big.” Which modal is correct?
- mustn’t
- can’t
- might not
2. Complete the sentence: “We left Melbourne six hours ago on the highway to Adelaide. We _____ be almost there by now.”
- can’t
- mustn’t
- must
3. Which sentence uses a modal of deduction correctly?
- “You mustn’t be tired — you slept nine hours.” (deduction)
- “She might know the reef well — she grew up in Cairns.”
- “He must to speak fluent Mandarin.” (deduction)
4. Choose the correct form for a PAST deduction: “The first Aboriginal Australians _____ the continent on foot, following seasonal food sources.”
- must cross
- must have crossed
- must crossed
5. Which modal expresses the WEAKEST level of certainty?
- must
- can’t
- might
▶ Reveal answer key
- b) can’t — For a strong negative deduction, use can’t. “Mustn’t” signals prohibition. “Might not” is possible but weaker — if you are drawing a confident conclusion from evidence (it is “far too big”), can’t is the right choice.
- c) must — Six hours of driving towards a destination = strong evidence you are nearly there. Must signals that confident deduction.
- b) — Option (a) uses “mustn’t” for negative deduction (wrong — use “can’t”). Option (c) puts “to” after “must” (wrong — bare infinitive only). Option (b) is correctly formed and logically appropriate.
- b) must have crossed — Past deduction about a completed action = modal + have + past participle. “Must cross” is present tense. “Must crossed” is ungrammatical.
- c) might — Might, could and may express weak deduction (it is possible). Must and can’t both express near-certainty — affirmative and negative respectively.