How to make deductions in English?

The most common way to make deductions in English is to use modal verbs like must, can't, might, may, or could. When you “make a deduction,” you are making an educated guess or conclusion based on the clues or information you have. The modal verb you use will show if you are very sure (must / can't) or not very sure (might / may / could).

Let's look at an example: Imagine that you baked a delicious chocolate pie, but then you return from the store and you find it has vanished. There are crumbs on the kitchen floor and your dog looks guilty. You can say:

• “Oh, no! The dog must have eaten my pie!”

  → You’re fairly certain it was the dog.

• “She might get sick!”

  → Chocolate can make dogs ill.

In this example, modal verbs play a crucial role in indicating the degree of certainty. Intrigued? Let’s explore more! It can’t be that hard to learn to make deductions in English!

One of the most common ways to make deductions in English is to use modal verbs of certainty and possibility. These modal verbs include must, have to, might, may, and could, and they allow us to express different degrees of certainty in our deductions. Here is a breakdown:

• Strong certainty:

  When we have strong evidence and are certain that something is true, we use must or have to.

  She must / has to be Jane’s sister because they look alike

• Impossibility:

  When we are certain that something is not true, we use can't or must not.

  Jane can’t / must not be home already because she barely left the office.

  It is uncommon, but you can use the full form cannot to make a deduction. However, we never use separate words to do this: ❌ can not.

• Possibility with some uncertainty:

  When we think something is possible but are not entirely sure, we use might, may, or could.

  Look at the gray clouds rolling in. It might / may / could rain soon.

• Possible negative:

  When we want to express that it is possible that something is untrue, we use might not or may not.

  The weather might not / may not clear up until tomorrow.

Deductions could be made not only about the present situation but also about past events. Let’s take a closer look at their constructions for each tense.

Making deductions about the present

To make deductions about the present, use the modal verb and the of the , following these two patterns:

• Positive deduction:

  modal verb
  must / has to / might / may / could

  root
  (of the main verb)

  My dog must be full!

  My dog might feel sick.

• Negative deduction:

  modal verb
  can('t) / must / might / may

  not

  root
  (of the main verb)

  My dog can't want more food now.

  My dog might not know that I am disappointed.

Look at the picture of Tom and see how many guesses you can make about him.

• Strong certainty:

  • Tom must / has to be a fireman.

  • It can’t be easy to be a fireman.

• Less certainty:

  • It might / may / could still take Tom a few more hours to put the fire out completely.

  • He might / may not get home until late tonight.

TipSound like a native:

In colloquial American English, have/has got to or have/has to are often used instead of must to make deductions with strong certainty. Take a look at the following sentences:

• Tom has got to be home by now. He took the five o’clock train!
  = Tom must be home...

• Look at the sky! It’s so dark and cloudy. There's got to be a storm moving in.
  = There must be a storm...

A couple of things to note when using these phrases:

• Have and has are often shortened with a contraction:

  • have → -’ve

  • has → -’s

• Got to is typically pronounced as "gotta".

Therefore, "Tom has got to" would usually sound like "Tom's gotta..."

Please note that this phrase is commonly used in casual conversations but may not be appropriate in a more formal context.

Making deductions about the past

When we make guesses about what happened in the past, use the modal verb, the have and the of the main verb, following these patterns:

• Positive deduction:

  modal verb
  must / has to / might / may / could

  have

  past participle
  (of the main verb)

  The dog must have eaten the pie.

  The dog might have thought it was his dinner.

• Negative deduction:

  modal verb
  can('t) / must / could / might / may

  not

  have

  past participle
  (of the main verb)

  She must not have known that chocolate was bad for dogs.

  She might not have realized that pie is not for dogs.

Check out our post on participles in English to review how to form the past participle!

Let’s make some deductions about the past using the picture below!

• Strong certainty:

  • It must / has to have snowed last night. The tree branches are covered with snow.

  • It can’t have been warm when the picture was taken.

• Less certainty:

  • Driving in the morning might / may / could have been dangerous.

  • Tom might / may not have driven to work this morning.

Important

There are actually two ways to make negative deductions with strong certainty about past events, but they have slightly different meanings:

• When we are certain that something did not happen, we can use must not have or can’t have (more colloquial).

• When we are certain that something was impossible, we use couldn't have.

Let’s compare the following sentences:

• Sarah must not have received my email, because she didn’t reply.

  → The speaker is certain that Sarah did not receive the email.

• Tom can’t have finished the entire pizza by himself.

  → The speaker thinks that it is very unlikely that Tom finished the entire pizza.

• Tom couldn’t have finished the entire pizza by himself; it’s too big.

  → The speaker is confident that it was impossible for Tom to finish the pizza alone.

In English, you can make deductions without using modal verbs by using instead. Let's explore some of these options:

• Likely and probably:

  These adverbs convey a strong sense of probability or likelihood. For example:

  • She’s been playing the piano since she was five years old; she’s probably very good at it.

  • There are dark clouds and strong winds. It’s likely going to rain this afternoon.

• Maybe:

  When you want to make deductions with low certainty, you can use the word maybe. For instance:

  • Maybe he’s stuck in traffic, that’s why he’s late for the meeting.

  • The traffic is unusually heavy today, so maybe there’s an accident on the freeway.

• Probably not and not likely:

  Use these phrases when you want to express negative deductions, including a low likelihood. For example:

  • She’s been absent from work for a week now, so she’s probably not feeling well.

  • Given his punctuality, it’s not likely he’ll be late for the meeting.

In addition to adverbs and modals, you can use to make deductions in English. These set phrases suggest beliefs based on the information you have. Some commonly used set phrases include “it seems (that)...” and “from what I can see / gather…”.

For example:

• It seems that they are having a disagreement.

• It seems that he’s really busy with his new project.

• From what I can see, she’s not very happy with the situation.

• From what I can gather, they’re planning a surprise party for her.

When making deductions in English, keep what we’ve covered in this post in mind:

• Use modals of certainty (must, can't) and possibility (might, may, could) to make deductions in English. Different modals indicate the level of confidence in deduction.

  • Use must when you have strong evidence and are certain something is true.

  • Use can't when you are certain that something is impossible.

  • Use might, may, or could when you think something is possible but are not entirely sure.

• Adverbs such as probably, likely, and maybe can be used to make deductions. They help express varying degrees of confidence or likelihood in a deduction.

• Set phrases like “it seems that”, “it looks like”, and “from what I can gather” can be used to make deductions based on the information available. These phrases often use the “it is” sentence structure and are valuable tools for making deductions effectively.

Let’s practice making deductions in English!