know

[therose]

Does "know + infinitive"
(as in "I knew to be careful" or "I knew to write a letter to him.")
mean

know how to + verb
("I knew how to be careful" or "I knew how to write a letter to him.")
)
or

know that one should + verb ("I knew that I should be careful" or "I knew that I should write a letter to him.")

[OddThomas]

Your second thought is correct: I knew to x = I knew that I should x.

Also important is I knew better than to x, meaning I knew not to x, or I knew that I ought not to x. (It does not necessarily mean that you made the right decision, however.:))I knew better than to enter the dark, scary mansion on a moonlit night by myself.