Writing clearly depends on thinking clearly, and thinking means making distinctions. The many rules and cautions of grammar serve to help us say what we mean, and among the more important of grammatical distinctions is the concept of tense. Tense means time, but just what is that?
Imagine this scene. You’re driving with a friend past a factory of some sort. You both have seen the place many times before, but as you wait at a light, your friend says, I never realized how big that building is. Straightforward enough, but why is the verb in the first clause (realized) past tense and the verb in the second clause (is) present tense? And why would the rules of grammar suggest we be aware of this change? The difference involves what is called the sequence of tenses, and that concept has the practical effect of helping us balance and clarify the ideas we’re expressing.
Any sentence that has more than one clause is presenting to the reader more than one thought. And anytime we have more than one of anything, the question of relationship will arise. Our example has two clauses, so how do those two thoughts, those two distinct assertions, stand in regard to each other? Are they both of equal importance? Is one meant to get more attention than the other? The speaker (or just as well a writer) of the sentence we’re considering has begun with a verb (realized) that carries the idea of thinking or knowing, and verbs of that sort trigger a grammatical construction called indirect statement. The speaker did not state directly, that building is so big, but rather framed, or nested, that one thought within a larger notion that means I didn’t think or I didn’t know. The one simple idea of a direct perception stands now in a subordinate relationship to second idea once removed: I never realized.
The effect of this, then, has been to subordinate one thought to another. By beginning his statement with an assertion about his mental perception, the speaker, at heart, wanted his friend to know first that he was thinking something; what he was thinking about—that a particular building was so large—was secondary to the fact that he had a thought in mind. That distinction is why the formal rules of grammar require that the verbs in an indirect statement follow (or sequence) each other in a prescribed way: when the main verb is past tense, the verb of the following clause should be past tense: I never realized how big that building was. Grammatical time, or tense, can differ from the absolute sense of time we normally live with, that there was a yesterday, is a today, and there will be a today tomorrow. In a number of constructions, grammatical time is relative, not absolute, and this preserves a way to make clearer distinctions. However large that building may be now, the speaker wanted to say something about his perception of it in the past, and so the verb of the clause which expresses that thought should be in the past tense.
Now there is (as you would expect) an exception to this rule, an exception that works to make an even clearer distinction in certain circumstances. If the subordinate clause of an indirect statement refers to a general truth, something which is and persists in the very nature of things, then the tense of the main verb should remain present. I might say, for example, I never realized how sunny it is in this part of the world, and by that I would be pointing to the enduring truth that the sun shines brightly, and probably often and warmly, in a particular geographical location. Whether I realized it in the past or realize it now is of less importance than the fact itself. One could argue, in turn, that the size of the factory is an enduring truth, at least for as long as the factory exists, and this, in fact, would be a way to justify the original sentence if that was the intent of the friend’s remark. And how could we know that? By the grammar he knowingly used and we knowingly understood.
We think about things (the two words are etymologically related), and things in language can be both objects in the world and thoughts in our mind. Writing clearly is a matter of organizing all those many objects and thoughts for our reader, including being clear about just when something was—or is.