One of the reasons it’s so important to be able to identify a clause is because every clause is a thought, and most of our writing for work or school or business involves putting thoughts together, explaining things for someone else. Thinking and explaining become more difficult than they already are when we lose our hold on the different ways we can connect our thoughts, and that’s why knowing how to recognize a thought in language is essential: we know where to find what we’re trying to say.
If I tell you, for example, that a friend of mine is an unusually generous person, I’m communicating a thought because you can identify something (a friend of mine) about which I am saying something (is an unusually generous person). In grammatical terms, I’ve produced a clause by putting a subject and verb together, and every clause is a thought. If that’s the only thing I want to say, I let that complete thought stand as a sentence in its own right, and so I’ve written a simple sentence.
But what if I go on to tell you something more about this generous friend of mine: If someone is in need, he helps them, no questions asked. This one sentence I’ve written now contains two thoughts, because there are two pairs of subjects and verbs (someone is and he helps). We do this quite often, of course, but this particular sentence begins with the word if, a subordinating conjunction, and that marks it as something called a conditional sentence. We write conditional sentences when we want to show how two thoughts are connected hypothetically, that is to say, when we build one thought to be the foundation or assumption for another thought.
But conditional sentences can become tricky when we realize that thoughts can be connected hypothetically in many different ways. The conditional sentence if someone is in need, he helps them means to communicate an actual truth about my friend, and it does so because the verbs in both clauses are in the indicative mood, that form of a verb we use when we intend to state a fact. The mood of a verb is distinct from its tense, so I can change the sentence to the past tense and still retain its factual character: If someone was in need, he helped them. Conditional sentences like these, whether in the present, past, or even future tense, express what is called a logical condition. We are to sense a regular, inevitable, almost mechanical connection between the two thoughts in a logical conditional sentence.
It may be, though, that you doubt my friend is so regularly generous. You might object with evidence of a time when he didn’t help someone you knew, to which I might then reply in his defense, if your friend had been in need, my friend would have helped. The connection between the two thoughts here is very different from our original statement, and we sense that different meaning because grammatically the verbs of the two clauses are in a different mood. Where our original logical condition stated fact with the indicative mood, in this type of conditional sentence, called an unreal, or contrary-to-fact, condition, the mood of both verbs is subjunctive. The subjunctive mood is used to express some degree of probability or outright denial that something is true, and when a conditional sentence has both clauses in the subjunctive, the latter is meant: it is not true that your friend was in need; otherwise, my friend would have helped him.
The topic of conditional sentences in English is not so much complicated as it is complex. It will marshal to order when we can command an understanding of mood and tense. Knowing how these are formed and combined will then allow us to work more systematically with the three types of conditional sentences (logical, ideal, and unreal). The complexity of it all lies not only in grammar, but also, and probably first, in the fact that our human experience is complex. But language follows life, and our attempts to simplify our language, or not try to understand how it works and what it implies, can end up blurring important distinctions. And that can make us very vulnerable to those who do not care about the truth.