“There must be a lot of pollen.”
You're sneezing a lot. Your friend asks you what's wrong. You think you're sneezing because of your allergies, so you say this.
There must be a lot of pollen.
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When you see evidence of something, or you have a strong reason to believe something, you can say "There must be ___". For example, when you hear music playing and people talking in the house next door to you, you can say:
There must be a party going on.
If your toilet doesn't flush correctly, you can say:
There must be something stuck in there.
Some English learners use the word "much" too much! In normal conversational English, "a lot of ___" is far more common than "much ___":
There was a lot of dust in the air.
My daughter always likes to put a lot of ketchup on her fries.
However, you can use "much" when you're saying "not much":
There's not much money left.
And it's good in the phrase "too much":
Sorry, I can't. I have way too much work to do here.
You can also use "much" for very, very formal positive sentences. When I hear a sentence like this, it reminds me of the Bible! It has that level of formality and age.
There was much rejoicing among the people.
"Many" is the same as "much". It sounds strange and old-fashioned in positive sentences, but normal in questions and negative sentences.
There were a lot of people there.
There aren't very many chairs left.
"Pollen" is the dust that flowers produce. A lot of people are allergic to pollen.