Each verb in a sentence should reflect the tense appropriate to the specific phrase rather than conform to the tense of another verb in the sentence. In each of the sentences below, a verb is not in the correct tense. Each example is followed by a discussion and a revision.
1. They are emblems of a simpler time, when everyone understood what it meant to be human.
What it means to be a human has not changed since the simpler time (though a universal understanding no longer exists), so although understood is correctly formed in context, the next verb should be in present tense: “They are emblems of a simpler time, when everyone understood what it means to be human.”
2. The financial sector underwent simulated terrorist attacks and cyberattacks as part of efforts to ensure that financial firms had good plans in place in the event of such crises.
The goal of the simulations is not to ensure that financial firms had, at a given time, good plans in place; it is to ensure that they continuously have the good plans, so the verb pertaining to “good plans” must be in the present tense: “The financial sector underwent simulated terrorist attacks and cyberattacks as part of efforts to ensure that financial firms have good plans in place in the event of such crises.”
3. If you thought marionettes were creepy, you’re not going to like this place.
The issue is not whether you, at one time, were disturbed by marionettes but no longer are (or, for that matter, whether marionettes were, at one time, creepy but no longer are); it is a matter of whether you have a discomfort with them that has existed, exists, and is likely to continue to exist: “If you think marionettes are creepy, you’re not going to like this place.”
I disagree with your analysis:
“If you thought marionettes were creepy…” is correct because this sentence is in the subjunctive mood, and “thought” is the correct verb.
“If you thought marionettes were creepy…” is a conditional clause because it might or might not be true.
A possible (mental response) to “If you thought marionettes were creepy…” would be “I think that marionettes are the coolest things since plain-paper fax machines!”
“Marionettes are even cooler than Winnie the Pooh!”
The subjunctive mood had stricken earlier, too.
“They are emblems of a simpler time, when everyone understood what it meant to be human.”
There is also a situation of uncertainty here — one that can be expressed well in writing: when “everyone” understood what it meant to be human.
Well, maybe “everyone” understood what it mean to be human, but on the other hand, it is highly probable that not “everyone” did.
There is an immense difficulty with the word “everyone”, because of — exceptions! exceptions! exceptions!
You would have to say, “Everyone thought that Genghis Khan was a monster….” because in the genuine truth of the matter, there were others who worshipped the ground that he walked on.
The same thing goes for Attila, Hitler, Mao, Stalin, Saddam Hussein.
Mother! Everyone else is doing it!
If everyone else cut his own head off, would you do it, too?
The word “cut” is in the subjunctive mood, and this might be a little difficult to see because the three principal parts of the verb are {cut, cut, cut}.
“If you thought marionettes were creepy…” Why do you think that is in the subjunctive mood?
Venqax, it is very simple. Either you do or do not think marionettes are creepy, or you are indifferent, and when the person made that statement, he/she had no way of knowing which.
“If you thought marionettes were creepy…” is clearly an unanswered hypothetical.
This is just like the statement “If you thought the world was flat, then you might go off the edge.”
[“you might go off the edge” has a double meaning, deliberately.]
More on the subjunctive mood:
“If you thought that Adolf Hitler was a madman, then just take a look at the history of Pol Pot…”
“If you thought that the Gulf of Mexico was a large body of water, then just take a look at the Pacific Ocean…”