Three Step English for Intermediate Learners - Learn #36 - Making Strong Assumptions - Grammar
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Let's look at the sentence pattern. This pattern is the structure that all of our examples will follow: subject + must be + adjective. This sentence pattern, subject + must be + adjective, is used to express a strong assumption or logical conclusion about a situation in the present.

When someone uses this pattern, they are not just guessing. They are making a statement that they feel sure about based on evidence or something they've observed. Let's go part by part. The subject is what the assumption is about. It's the person or thing being described. Must be is a modal construction. The word must expresses a high level of certainty.

When combined with be, it points to a condition or state that we strongly believe is true now. The adjective phrase comes after that. It describes the current situation or quality of the subject. Things like mood, behavior, condition, or performance. So the full pattern, subject plus must be plus adjective, is a way to say, I'm confident that this is true right now.

Let's see how a line from the dialogue follows this pattern. You must be so excited. Let's break it down. The subject in this sentence, you, that's the person we're talking about. Next, we have must be. This is a modal verb phrase. Must is used here to show a strong assumption or logical conclusion.

When we say "must be," we're expressing that based on what we know or observe, we feel confident that something is true. Then comes the adjective phrase: "so excited." This describes the current state or condition of the subject. So when we put it all together, "You must be so excited," we're using this structure to strongly assume that the person is excited about her road trip.

Now let's look at some speaking examples. It must be boring to sit at a computer all day. Can you see how the pattern applies here? This sentence uses the pronoun "it" as a subject referring to the experience of sitting at a computer all day. "Must be boring" is the adjective phrase, and it expresses what the speaker thinks about the situation.

It's a variation of the pattern where "it" stands in for a general idea or activity. Next, "Seafood in Japan must be so fresh." Here we have a clear noun, "Seafood in Japan." The speaker is making a confident guess, probably based on what they know about Japanese cuisine. "Must be so fresh."

is a verb phrase that includes the modal verb phrase "must be" and the adjective phrase "so fresh." This fits the full pattern: subject + must be + adjective phrase. Let's try one more. You must be so nervous about your exams. You is the subject. The speaker is making a strong assumption about how the person feels.

"Must be so nervous about your exams" is a verb phrase that includes the adjective phrase "so nervous about your exams." Again, this follows the pattern exactly. Another one: "It must be so exciting to be a professional baseball player." In this example, "it" is the subject and means the experience of being a professional athlete.

Like before, this is a situation-based subject, and the pattern of "must be" plus adjective/phrase still clearly shows the speaker's assumption. It's a slightly different structure, but used in the same way to express a strong feeling or judgment.