I've recently developed a habit of wondering (often to the point of frustration) how and when best to combine the simple past and simple present tenses in a logical manner. I've looked at a number of guidelines establishing when to make the transition from one to the other, yet the confusion hasn't been dispelled. I understand that it's prudent to use the present tense in a dependent clause following an independent clause with a verb in the past tense where a condition is considered timeless or still true, but I can't help but feel that strictly following that criterion would result in some awkward constructions, for example this relatively simple one:

"I never knew that you have such a callous heart."

If the person to whom this sentence is referring is still in possession of a callous heart, wouldn't this be a sensible construction? Yet the simple past here (had) would seem much more natural. Why is that, and are there any hard rules governing exactly when to keep the tenses in the independent and dependent clauses consistent? Thank you greatly for any assistance.
The rule is as you state it: if the condition still appertains now, then the present tense is also possible. I say 'also' because the natural tendency of the native speaker is to regress the verb of the independent clause. That is why it sounds more 'natural' in past tense.
purveyor"I never knew that you have such a callous heart."
knew and thought in the main clause are almost never followed by a subordinate clause in the present tense.

You'll rarely go wrong by backshifting, regardless of the verb in the main clause. Save yourself the trouble of determining who has a callous heart and when they had it. You'll drive yourself crazy. Backshift and be done with it! This is not advanced calculus. Emotion: smile