(Which/What/That/Things) we saw surprised us. Which is the correct answer?
Both of the following are complete sentences.
What we saw surprised us.
Things we saw surprised us.
Neither “which” nor “that” will make a complete sentence.
Sentence 1 above, I feel, is most usual to say that something surprised you. Whether that thing is singular or plural.
If you want to specifically indicate that there were multiple things that surprised you, use sentence 2.
"What we saw surprised us," is correct, but you'd probably write "that which" instead of "what" for an exceedingly formal context. "Things we saw surprised us," is also correct, but usually you would hear something like "the things," "some things," or "things that," since "things" is terribly vague without a touch more specification.