grammaticality - speaking about conditional past event using had (verb)... would (verb)

If the author had spent any time in China, they would know that an “engineer” in China is NOT the same as an “engineer” in the USA. This is a mistranslation. An “engineer” in China is equivalent to a “technician” in the USA.

Is this correct, or should would be followed by have known?

Please explain the rule.

Answer

If the author had spent time in China, according to the quote, they would not only have learned that an engineer is different there, but would presumably retain that knowledge today... hence the present tense.

You might object that the author wrote the mistaken passage in the past, and should therefore be referred to using the past tense, but it is a common conceit to talk about literary works and similar things in the present tense: "In Julius Caesar, Shakespeare writes about...", etc.