Let’s tackle, or at least try to wrestle to the ground, a slippery topic in English grammar: the conditional sentence. Entire volumes could be (and have been) written on the subject, so here in the space of eight hundred words or so, our goal is just twofold: to get a bird’s eye view of the topic and to look more closely at one part of it that causes some trouble, the contrary-to-fact condition. There are subtleties here to appreciate.
To say something conditionally means to say that something is certainly true, or perhaps true, or certainly not true—depending on certain circumstances, called conditions, which are also given in the sentence. To say if it is raining, the stairs are slippery means that on the condition of there being rain, it is certainly true, it is certainly a fact, that the stairs I am referring to will be slippery. If we change the verbs just a bit and say, should it be raining, the stairs would be wet, we have introduced some doubt into the assertion: perhaps it is true that the stairs are slippery, but then again perhaps not, because there is some doubt whether the conditions that would certainly make the stairs slippery has in fact been fulfilled. Finally, if we change the verbs one more time to say if it were raining, the stairs would be wet, we are positively asserting that it is not true that the stairs are slippery, because the condition, that it is raining, is now understood as a supposition, not as a certain or even probable fact.
That is a synoptic view of a conditional statement. This kind of sentence, then, has two parts, a conditional clause and a principal clause. The conditional clause (technically in both grammar and logic called the protasis) generally begins with the conjunction if, and the principal, or concluding, clause (technically called the apodosis) says what ensues, given the conditional clause connected with it. The order of the two clauses is not grammatically important (though rhetorically very important), and sometimes the conjunction if is omitted, the writer assuming the reader will supply it. If the if-clause begins the sentence, a comma must follow it.
Now let’s concentrate on the contrary-to-fact condition, because this involves the difference between the assumption of fact and the outright denial of it while imagining it true. If not observed, this distinction can cause much pointless misunderstanding, because conditional sentences, strange to say, make no actual assertion about anything at all; they only say something about the connection between two ideas. What is factual in the statement if it is raining, the stairs are slippery is the inevitability of the slippery steps in the presence of rain. Whether or not it is in fact raining at the moment is something the writer does not know, for if he did, he would say, it is raining and so the stairs are slippery. That sentence is not conditional but merely the regular combination of two indicative (that is, factual) assertions. The latter is the result of the former, and the former is not a necessary condition of the latter, at least as far as the sentence is concerned. In reality, however, that is almost certainly the case.
Compare that now with the contrary-to-fact sentence If it were raining, the stairs would be wet. How do we know upon reading that sentence that the writer means to assert that at the moment it is not raining, but something would result from the rain if we imagined, or hypothesized, that there was rain present? The answer lies in the way the verbs are used. Notice that in this contrary-to-fact sentence, both of the verbs, were and would be, appear to be in the past tense, but are still referring to the present time. Read the sentence again and you will sense that right away. Compare that now with the factual statement, if it is raining, the stairs are slippery and you’ll see that both verbs there, is and are, appear to be in the present tense.
The grammatical sleight of hand that makes the contrary-to-fact condition possible is this: when the past tense form of a verb refers to the present time, it constitutes what is called the subjunctive mood of the verb. That subjunctive mood is used in contrary-to-fact conditions, but not in factual, or what is sometimes more accurately called logical, conditions (though it is also used in the second kind of conditional sentence that involves the verbs should and would, past subjunctive forms of shall and will respectively).
Making the past tense presently open up an imaginary world is an ingenious linguistic device, a kind of magical open sesame or once upon a time, whereupon we can find ourselves in the unhindered world of the imagination, what some still believe to be the priceless pearl of human beings—and being human. There in that immaterial world (or is it?) we can think out things before getting caught in the predicaments of factual existence, and save ourselves much trouble indeed. Such is the subtlety of grammar and the shades of meaning it can convey.
***