A. Modals for speculation – present and future
1a. Will/Won’t – used for things in the future considered to be facts or certain.
“She’ll be home in a few minutes.”
“The eclipse will happen on July the 3rd.”
1b. Will/Won’t – used for the present when we are certain based on habit, character, quality etc.
“Are the cookies done?” > “No, they won’t be ready now.”
“Is that the phone? It’ll be the garage about my car.”
2a. Must – used for what we believe to be the most logical explanation of something. There is evidence, but it isn’t as sure as ‘will’
“The lights are off in his apartment. He must be out.”
“What? You must be joking! You aren’t really getting married, are you?”
2b. Can’t – used for things that are logically not likely. Do not use ‘mustn’t’ in this situation. ‘Can’t’ is the opposite of ‘must’ for logical explanations.
“The lights are off in his apartment. He can’t be home.”
“What? You can’t be serious! You aren’t really getting married, are you?”
3a. May, may not – used for showing the possibility of something happening now or in the future.
“It may rain later. The forecast gave a 60% chance.”
“I may not have time to go to the bank. I’m pretty busy.”
3b. Might, might not – used the same way as ‘may’ but some people feel it is a little less sure.
“I might have a cold. My throat is a bit sore. But my nose feels fine.”
“He might not know the address. I forgot to ask him if he knew it.”
4. Should, shouldn’t – used for what we think can be expected to happen, but not in a negative way. We hope it happens this way. ‘Should’ can also be used in a conditional sentence to show the expected result if nothing goes wrong.
“He should be here soon. He just texted me and said he was 2 minutes away.”
“He shouldn’t be long. He just went to the restroom.”
“She should get sick if she drinks that dirty water.”
“You should do well in the test if you have studied hard.”
5a. Could – used for showing the possibility of something happening now or in the future. A little less sure than ‘might’, often a little doubtful.
“Well, it could snow, I suppose. But I don’t think it’s cold enough.”
“That book could be a bit boring. The cover is really dull!”
5b. Couldn’t – this is not the opposite of ‘could’ in 5a. In that situation ‘might not’ is better. ‘Couldn’t’ is used in a similar way to ‘can’t’ for logical conclusions, but it is more disbelieving or doubtful.
“Good news! It couldn’t might not rain. The dark clouds are all moving away. I think it’ll be sunny.”
“He couldn’t be her husband, could he? He’s far too old!”
“That’s strange. The lights are on, but surely, he couldn’t be home. He was still in a meeting when I left.”
6. Can – used for general beliefs for what is generally true or logically possible. This is only used for the present. For future situations use ‘will be able to’ or ‘will be possible to’.
“Driving in the snow can be dangerous if you don’t have experience.”
“The weather can be very unpredictable here.”
“Electric cars keep getting better. We’ll soon be able to drive from Tokyo to Fukuoka on a single charge.”
B. Modals for past speculation
All of the above modals can be used for speculating about the past by using ‘have’ and the past participle (“The movie can’t have been very good – he fell asleep.”) except ‘can’. ‘Would’ is also possible.
“He will have finished dinner. He always eats early.”
“A woman answered my phone? That would’ve been my sister.”
“She may have forgotten about the meeting. She was sleepy when I told her.”
“The tennis match might not have started yet. Rain stopped play earlier today.”
“The company should’ve delivered the package a few hours ago.”
“It must have rained. The road is wet.”
‘Can’ is only used in questions or with words like ‘hardly, only, or never’
“Who can he have been talking to?”
“He can never have done it by himself!”