Then--shows a sequence: I passed my driver's test and then bought a car.
Than--draws a comparison: I am older than my brother is.
That--refers to something at a distance: My brother lives in that purple house across the street.
That--introduces essential information: The report that John wrote yesterday had three errors.
Which--introduces nonessential information: I graduated from USF, which is in San Francisco.
This--refers to something near at hand, here and now: My brother lives in this neighborhood.
This--refers to something previously referenced: One of my brothers is a lawyer. This brother lives in L.A.
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