Linda: Mom, I'm really worried.
Mom: Why? What's wrong?
Linda: I can't find my schoolbag.
Mom: Well, where did you last put it?
Linda: I can't remember! I attended a concert yesterday so it might still be in the music hall.
Mom: Do you have anything valuable in your schoolbag?
Linda: No, just my books, my pink hair band and some tennis balls.
Mom: So it can't be stolen.
Linda: Oh, wait! I went to a picnic after the concert. I remember I had my schoolbag with me at the picnic.
Mom: So could it still be at the park?
Linda: Yes. I left early, before the rest of my friends. I think somebody must have picked it up. I'll call them now to check if anybody has it.
It might still be in the music hall.（可能还在音乐厅。）It can't be stolen.（不可能被偷。）Could it still be at the park?（可能还在公园吗？）Somebody must have picked it up.（一定有人捡到了。）must > could/might > can't 表示肯定→可能→不可能。
We live in a small town and almost everyone knows each other. It used to be very quiet. Nothing much ever happened around here. However, these days, something unusual is happening in our town. Victor, a teacher at my school, is really nervous. When he was interviewed by the town newspaper, he said, "Every night we hear strange noises outside our window. My wife thinks that it could be an animal, but my friends and I think it must be teenagers having fun. My parents called the policemen, but they couldn't find anything strange. They think it might be the wind. I don't think so!"
It could be an animal.（可能是动物。）It must be teenagers.（一定是青少年。）It might be the wind.（可能是风。）不同推测程度：must(肯定)>could(可能)>might(也许)。
Victor's next-door neighbor Helen is worried, too. "At first, I thought that it might be a dog, but I couldn't see a dog or anything else, either. So I guess it can't be a dog. But then, what could it be?" One woman in the area saw something running away, but it was dark so she is not sure. "I think it was too big to be a dog," she said. "Maybe it was a bear or a wolf."
It was too big to be a dog.（它太大了，不可能是狗。）too + adj + to do = 太……以至于不能……
Everyone in our town is feeling uneasy, and everyone has his or her own ideas. There must be something visiting the homes in our neighborhood, but what is it? We have no idea. Most people hope that this animal or person will simply go away, but I do not think that is going to happen. The noise-maker is having too much fun creating fear in the neighborhood.
There must be something visiting the homes.（一定有什么东西在访问各家。）The noise-maker is having too much fun creating fear.（制造噪音者太享受制造恐惧了。）must be + doing = 一定在……（对正在进行的动作的推测）。
Stonehenge, a rock circle, is not only one of Britain's most famous historical places but also one of its greatest mysteries. Every year it receives more than 750,000 visitors. People like to go to this place especially in June as they want to see the sun rising on the longest day of the year. For many years, historians believed Stonehenge was a temple where ancient leaders tried to communicate with the gods. However, historian Paul Stoker thinks this can't be true because Stonehenge was built so many centuries ago. "The leaders arrived in England much later," he points out.
Another popular idea is that Stonehenge might be a kind of calendar. The large stones were put together in a certain way. On midsummer's morning, the sun shines directly into the center of the stones. Other people believe the stones have a medical purpose. They think the stones can prevent illness and keep people healthy. "As you walk there, you can feel the energy from your feet move up your body," said one visitor. No one is sure what Stonehenge was used for, but most agree that the position of the stones must be for a special purpose. Some think it might be a burial place or a place to honor ancestors. Others think it was built to celebrate a victory over an enemy.