I’m afraid that you didn’t quite get the essence of the sequence-of-tenses rule. I’m saying this because this paraphrase of yours of that rule is imprecise, almost trivial: “… it is therefore safe to say that the main clause, usually, can be of any tense and the tense of the subordinate clause more often than not agrees with the tense of the main clause.” Your use of the qualifiers “usually” and “more often than not” makes the prescribed course of action very fuzzy and not actionable at all. This is the danger of attempting to overgeneralize the sequence-of-tenses rule, which clearly operates by exception: “When the operative verb of the main clause or independent clause
is not in the past tense or in the past perfect tense, the operative verb of the subordinate clause or dependent clause can be in any tense that conveys meaning accurately.”
Conversely, when the main clause isn’t in the past tense or past perfect tense, it could only be in the present tense or in the future tense, so a valid restatement of the sequence-of-tenses rule would be this: “When the operative verb of the main clause or independent clause is in the present tense or the future tense, the operative verb of the subordinate clause or dependent clause can be in any tense that conveys meaning accurately.” The tense of the subordinate clause will then be determined by the difference between the time expressed in the independent clause and the time expressed in the subordinate clause.
That’s admittedly a tough concept, so it needs particulars and specifics to be understood. I suggest that you study very carefully
the sequence-of-tenses chart provided by the CCC Foundation’s Guide to Grammar and Writing. To clarify here how that concept works, I’ll focus only on the situations when the main clause is in the present tense, after which you can study the rest of the other possible situations listed in that chart. It’s also very important to keep in mind that complex sentences don’t exclusively use “when” to link the subordinate clause with the main clause. Indeed, depending on the time difference between them as well as the intended sense, the subordinating conjunction can range rather widely among these common choices: “before,” “after,” “during,” “as,” “that,” “while,” “because,” and “although.” We must free our mind from the wrong notion that only “when” can be used or is suitable in such situations.
Now, when the main clause is specifically in the present tense, there will be four possibilities for the tense of the subordinate clause depending on the intended sense, as follows:
1. For same-time action as in the main clause, the present tense can be used for the verb in the subordinate clause. Examples: “I get thrilled
when I watch action movies.” “She is here
although she thinks her presence isn’t necessary.”
2. For an action earlier than that in the main clause, the past tense can be used for the verb in the subordinate clause. Example: “She’s angry
because her boss berated her.” “I think
that she already left.”
3. For an action in the main clause that extends from some point in the past to the present, the present perfect tense can be used for the verb in the subordinate clause. Example: “They doubt
that he has met his sales quota.” “Why is he making all that fuss
when has not met his sales quota?”
4. For an ongoing action in the main clause, the future tense can be used for the verb in the subordinate clause. Example: “The police knows
when the detainee will be released.” “The father expects
that his son will finish college next year.”
You said that your understanding is that the subordinate clause can take any verb tense depending on the meaning that the speaker wants to convey. This overgeneralization is definitely incorrect, as can readily be seen in this example: “She was asleep when I leave tomorrow” (past tense/future tense). The statement is obviously illogical and senseless. Indeed, only when the belief, declaration, or utterance by the speaker is in the present tense can the subordinate clause take any verb tense, as in the following examples: “She
believes that the country
needs a visionary leader” (present tense/present tense). “We
think that the company
had missed its sales targets” (present tense/past perfect tense). “They
expect that the company
will meet its midyear its sales targets” (present tense/future tense). Again, I would like to emphasize that it’s dangerous to make other generalizations about the sequence-of-tenses rule beyond those specified by the general rule. We must always keep in mind that the sense and logic of the statement—not the tense individually taken by the main clause and subordinate clause—are the overriding determinants in constructing complex sentences.
This other example of the past tense/past tense combination for the subordinate clause and main clause is not only justified but absolutely correct: “When I
went to bed last night, I
heard a noise outside my window.” But again, I must warn you against making the generalization that “since the action of going to bed precedes the hearing of the noise, the subordinate clause sets the time in the past,” and that “if the speaker’s focus is the time the action took place, then the verbs should be in the past tense.” This generalization is easily rendered false by these perfectly acceptable variations of your sentences: “When I
go to bed every night, I
hear a noise outside my window” (present tense/ present tense). “When I
go to bed at night, I
would hear a noise outside my window” (present tense/frequent or customary action).
As a rule then in constructing complex sentences, I can’t overemphasize this advice: Go for the sense and logic of the statement and choose the most suitable subordinating conjunction rather than unduly focus on the particular tense of the main clause and subordinate clause.
