>>769
"What he told me was good."

I guess this "what" is a pronoun that means "the thing/fact which..."
Japanese usually say "こと koto," which is kind of a general-purpose pronoun.

What he told me = 彼が 私に 言った こと (kare ga watashi ni itta koto)
What she did = 彼女が やった こと (kanojo ga yatta koto)

The phrases placed before こと are assumed as adjective parts. So "kare ga watashi ni itta koto" forms a noun clause, thus you can use this part as a subject.

A word for word translatin of "what he told me was good" is "彼が 私に いったことは よかった."
This translation is gramatically correct and understandable but sounds somewhat strange because the adjective "よい" (good) is usally (not always) used before noun. Japanese says "彼は 私に よいことを いった (He told me a good thing)." However, "彼が 私に いったことは 正しかった (What he told me was right/correct/true)." is no problem.

"こと" is used to refer to an abstract, an idea, or a fact. If you refer to a physical object such as a book, a building, an animal, you can use "もの mono" (thing/object).

Example.
"What he bought (the thing he bought) was very expensive."
= 彼が 買った もの は とても 高価 だった.

"こと" and "もの" are sometimes interchangable.
"What the novelist wrote"
= その小説家が 書いた こと/もの
In this case, while こと tends to mean the storyline or the idea the novelist wrote, もの means the book. But the difference between them is not clear.