The distinction between clauses and phrases is indeed a muddy one. I've shown before how it confused the Longman Dictionary of Language Teaching and Applied Linguistics. Apparently, its authors are not alone in their confusion.
The reality of the situation is that the words are used differently by different people, or even sometimes by the same person. And that's fine. Just as long as we can figure out who's using which word what way.
Traditionally, then, a phrase is merely any contiguous group of words that has some connection in expressing a concept, but which is not in itself a clause. A clause, on the other hand, is a whole simple sentence or one part of a complex or compound sentence that contains a subject and a predicate.
These two concepts are generally distinct enough that the distinction is maintained, but things can get messy. What does it mean to contain a subject? Imperatives are thought to be clauses and are said to include implied subjects: you. But then a predicate adjunct such as speaking slowly in Speaking slowly, he explained why he disagreed also has an implied subject: he. Traditionalists, though, would not consider speaking slowly a clause.
Firsten tries to explain this traditional idea of clauses vs. phrases and how it overlaps with the idea of being finite or non-finite.
"Non-finite clauses are the same thing as "phrases." In addition, finite clauses are the same thing as "clauses." The terminology underwent a change some years ago, and now most grammarians use the terms finite and non-finite clauses... A finite clause must have a subject and a verb that carries tense."This rule of thumb isn't very useful as it ignores the possibility of phrases that contain no verbs at all, while mostly just switching one label for the other without adding any clarity.
So what does it mean for something to be finite? The idea of finite verb has been with us since at least the late 18th century when Lindley Murray wrote, "Finite verbs are those to which number and person appertain." This explains the reason for the label: if something is finite, it has certain limitations. A verb like runs cannot go together with just any old subject; I goes will not do. The verb go in to go, on the other hand, is not limited in this way. That is to say, it is non-finite.
Firsten's explanation is at odds with Murray's use. Murray applies the concept to verbs, where Firsten applies it to entire clauses. But given that English subject-verb agreement occurs only with present-tense verbs and with past-tense be, it seems that they agree that it applies when the verb carries the tense. Firsten doesn't stop there though. He says,
"Come with me to the next TESOL convention is also a finite clause even though the subject you is hidden and the verb is in the imperative form."Yet, there is no tensed verb here and subject-verb agreement does not apply.
The Cambridge Grammar of the English Language takes a position rather closer to Firsten's than Murray's, albeit one which is much clearer while at the same time admittedly unorthodox. Like Firsten, the CGEL applies the concept to clauses, not verbs. But it is silent about a subject. Rather it includes the following as finite clauses:
- clauses headed by tensed verbs (i.e., present tense or past tense; there is no future tense)
- clauses headed by plain-form verbs in the imperative or the subjunctive.