Common grammarians see phrase as a bigger construction than word and smaller than clause, although they do not reject that a single phrase can consist of only one word. In this regard, a word like fishes can be considered as a word and as a phrase at once. How to judge that the word is a word or a phrase? I’d like to say that “that word is a phrase if it is in a certain relationship with other words in a certain construction.” See the example:
Fishes [this is totally a word]
John fishes [this is totally a verb phrase]
The single word fishes or the other single words are only words and never to be judged as a phrase until they are placed in certain construction, mostly in a clause.
This is different when we take example of a phrase which consists of more than one words like big house which can stand alone as a phrase (noun phrase) without being in a certain construction. Thus, what makes it different? One word is a phrase if it is in a construction, and more than one word phrase remains a phrase whether or not it is placed or used as an element in a construction.
As a category, a phrase possesses a higher level position than a word class, and it possesses a lower level position than a function. Some new comer in grammar may be confused with this statement, but actually it is basic. This statement is a conclusion that as a phrase a word requires a position as a constituent, which is not required by a word as a word. This is so making sense. The overlapping between the terms “word” and “phrase” for a single word is now overcome. That is, the grammarians or syntax experts should point on a clear cut between those terms. The new language students may deal with difficulties when the teacher asks “when can we say that fishes is a word and a phrase?”
Now we look at the example of more than one word phrase like big house above. The construction is a phrase although it is not in a bigger construction like we will buy the big house. How could? It is because the two words fulfill the basic requirement to be a phrase: the existence of a head and a modifier. Moreover, it is hard to say that big house is a word, it is two words. The function of the attributive adjective big which modifies the head house has fulfill the requirement. Now, if we omit the modifier, then the word house returns to be a word, not a head of a phrase anymore.
The overlapping happened in phrase also happens in clause. It is easy to say that “My mother will come tomorrow if it is not raining” is not a clause but a sentence. However, to say that “my mother comes tomorrow” as a sentence is still debatable. How could? Because it is also a clause. How to differentiate a clause from a sentence? It is not necessarily the matter since a subordinate clause is totally a clause and cannot be a sentence. The matter is that a main clause also functions as a sentence, a simple sentence.
A clause is smaller than a sentence. Although a main clause can play role as a sentence, clause remains clause, clause is not sentence. Take a look on an example below:
A. My mother comes tomorrow. [a sentence]
B. My mother comes tomorrow if it is not raining. [a clause/a main clause]
There is a shared property between a clause and a sentence: they fulfills SV/SVO/SVOCA formulation. SVOCA stands for Subject-Verb-Object-Complement-Adjunct. This is how we judge whether a construction is a clause or a sentence: If my mother comes tomorrow is placed in a bigger construction like the example B above, it is a clause. If it is independent like the example B then it is a sentence.
Clause A + Clause B = Sentence A
Clause AB – Clause B = Sentence A, but
Clause A ≠ Sentece A
The status of the clause A changes to be sentence A. It is not a clause and a sentence at once. This is rational since a clause is called main because there is a subordinate clause related to it. If there is no subordinate clause related to it, the status as a main clause replaced to be a sentence, a simple sentence.
One may ask what about main clause + main clause? Is it possible that there is not a subrodinate clause? Like:
My mother comes today, my father comes tomorrow.
In spoken communication, this happens. However, there is at least an “and” between both clauses. The “invisble” conjunction clues that both clauses are coordinated one to another, and one of them plays role as a main clause, and the rest does not.
Clause inside Phrase
Clause is a bigger construction than a phrase; at least that is common understanding we have about that. However, the relative clause and content clause are the most clauses used as an element in a phrase.
The letter you sent last month was delivered by the postman.
The letter I asked you (to) send was delivered by the postman.
The underlined parts are noun phrases. Those noun phrases consist of a clause. The clause functions as the modifier. In the two phrases, the head is letter, and the rests are the modifiers. By looking at this, the “bigger construction” does not mean “more words inside” but the function in the construction. May be. it is more accurate to mention “higher” rather than “bigger” to differentiate word, phrase, clause, and sentence. Sentence is higher than clause, clause is higher than phrase, and phrase is higher than word. It is higher based on the function, not bigger based on the word amount in it.
For example, the underline parts above are phrases consists of clauses. However, the clauses are functioning as a part of a phrase, the modifiers of the phrase head. Therefore, if a clause functions as a modifier, then it is even smaller than a phrase (just a part of it!). And if a phrase functions as a subject in a sentence, although it consists of 100 words, it is never “bigger” than a sentence, even not bigger than a simple clause (clause simplex).