Hello, fellas. Today we will learn how to use which to modify a whole sentence. Which is usually used in a clause modifying a noun or a relative clause (an adjective clause)
(More on relative clauses: https://englishtips4u.com/2011/11/08/engclass-relative-clause/ and https://englishtips4u.com/2011/11/09/engclass-relative-clause-2/)
A sentence is modified by which in informal and spoken English. It is not generally deemed appropriate in formal writing. If it is written, a comma comes before it to reflect a pause in speech.
1) Liverpool won the UEFA Champions League. That did not surprise me.
Liverpool won the UEFA Champions League, which did not surprise me.
2) We are facing a long dry season. This is too horrible.
We are facing a long dry season, which is too horrible.
The pronouns that and this refer to the ideas of the previous sentences “Liverpool won the UEFA Champions League” and “We are facing a long dry season”. Then, they are replaced by which.
Betty Schrampfer Azar, Understanding and Using English Grammar: Third Edition