點解要加that係think後面?

<Subject> think that <clause>
A clause can be a complete sentence by itself. The word "that" can be omitted without change of meaning.

For example:
I think that people buy more healthy food nowadays.
I think people buy more healthy food nowadays.

Do you think that John's answer is correct?
Do you think John's answer is correct?



EXCEPTION: However, sometimes "think" is not followed by a clause. For example:
a. You think so? (correct - casual)
b. You think that so? (incorrect)
c. You think that? (correct - casual, but not the same as a. Here "that" is an indicator, referring to something else.)
d. You think that's so? (correct, but a. and d. are not the same. a. does not contain a clause; whereas d. contains a clause.)

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因為加左 that 之後

後面就可以用complete sentence

即 S + V + O