Epistemic Must (Logical Deduction)

This isn't the "must" that your teacher uses when they say, "You must finish your homework!" That kind of must is a rule or an obligation. Today, we are talking about logic. Think of it like a math problem with words. Clue A + Clue B = must. When you use must this way, you are telling the listener that you have evidence. If you see a lot of people carrying pizza boxes in your apartment building, your brain does a quick calculation: "Pizza boxes + many people = party." So, you say, "There must be a party upstairs." You are about 95% sure. It is like a grammar traffic light that is stuck on green—you are ready to go with your guess because the evidence is so strong. It’s the Sherlock Holmes method of speaking English. You aren't just talking; you are thinking out loud.

1. 1Using this grammar is like building with LEGO blocks. You only need three main pieces to make a perfect sentence.
2. 2The Subject: This is the "who" or "what" of the sentence (like I, you, he, she, it, we, or they).
3. 3The Helper: This is our star word, must. Here’s the best part: it never changes! It doesn't care if the subject is he or they. It stays exactly the same. No extra letters allowed.
4. 4The Action: This is the base form of the verb. Think of it as the "naked" verb. No to before it, no s at the end, and no ing.
5. 5For example, if you see someone with a very expensive gold watch, you say: He (Subject) + must (Helper) + be (Action) + rich. Notice we don't say "He musts" or "He must to be." It’s so easy it almost feels like cheating! Just keep it simple: Subject + must + Verb.

Use must whenever you have a "smoking gun"—a piece of evidence that makes you very sure. Here are some real-world moments where you can shine:

• Ordering Food: You are at a new restaurant. Every table is full and people are smiling. You turn to your friend and say, "The food here must be delicious!"
• Job Interviews: You see a man walk out of an office. He is wearing a fancy suit, everyone is shaking his hand, and he looks very happy. You whisper, "He must be the new boss."
• At Home: You hear a loud "meow" coming from the kitchen. You think, "The cat must be hungry again." (Let’s be honest, the cat is always hungry).
• Weather: You see someone opening an umbrella. You say, "It must be starting to rain."

In all these cases, you are connecting the dots between what you see and what is probably true.

Don't use must if you are just making a wild guess without any clues. If you have no idea why your friend is late, and you’re just guessing based on a dream you had, use might instead. Must is for when you are 90-100% sure. Also, don't use it for things you know for a fact. If you are literally standing in the rain and you are soaking wet, you don't say "It must be raining." You just say "It is raining!" We only use must when there is a little "jump" from the clue to the conclusion. Finally, don't use it for negative guesses. If you are sure something is *not* true, must is not your friend. We have a different word for that, which we will see in the next section.

Let’s talk about the "Mustn't Trap." This is where most learners trip and fall. If you want to say, "I am sure he is NOT hungry," you might want to say, "He mustn't be hungry." Stop right there! In English, mustn't is only for rules (like "You mustn't smoke here"). For a logical guess that something is *not* true, we use can't.

• ✗ Wrong: He mustn't be at home; the lights are off.
• ✓ Correct: He can't be at home; the lights are off.

Another mistake is the "to" ghost. Don't let the word to sneak into your sentence like a cat at a tuna convention. Never say "It must to be true." It’s just "It must be true." Also, remember that must is a helper verb. You don't need do, does, or am/is/are before it.

Think of certainty like a volume knob on a radio.

• Might / May: This is low volume (50% sure). "It might be him, but I'm not sure."
• Must: This is high volume (95% sure). "It must be him! I recognize his hat."
• Can't: This is the "mute" button for things that are impossible. "It can't be him; he is in Paris right now."
• Should: This is for expectations. "He should be here by 5:00" means that was the plan. "He must be here" means you hear his voice in the hallway. Choosing the right word tells the other person exactly how much you trust your own brain. It’s like choosing between mild, medium, and "English-teacher-hot" levels of certainty.