Is "This is the only mammal that can fly" an independent clause?

Question

We were taught that an independent clause must have a complete thought/idea.

This is the only mammal that can fly.

How can "This is the only mammal" be treated as an independent clause? It was a grammar lesson and there was no context there. In this situation how can I decide that it is an independent clause? Though for sure, there should be an independent clause in a correct sentence, and the clause starting with 'that' is not independent. But by this way, I can't fit it with the rule that every independent clause must have a complete thought. A simple sentence must have an independent clause. If the sentence has no independent clause how we can call it a sentence? Can even two dependent clauses form a sentence?

Answer 1

In the sentence: "This is the only mammal that can fly," "This is the only mammal" is indeed the main clause. The term "independent" may in fact be misleading because main clauses often contain elements that link them with subordinate or dependent clauses, and if we take the latter ones out, the main clause will appear to be incomplete.

In the sentence at issue, that word within the main clause establishing a liaison with the subordinate one is, oviously, the adjective "only".

In this other sentence:

• This bird is so strong that it can fly for hours,

the main clause is "This bird is so strong," but once again we find an element that links it with the dependent clause "that it can fly for hours." That element is the adverb "so."

The question is, then, how to spot the main clause in a sentence. You'd better forget about calling it independent. The main clause will be the one that is not introduced by a subordinating word, be it a relative pronoun or adverb or a conjunction.

Notice that the relative (dependent) clause "that can fly" is introduced by the relative (subordinating) pronoun "that": then, that is the dependent clause and whatever remains is the main clause.

In the other example I provided, the adverbial (dependent) clause of result "that it can fly for hours" is introduced by the subordinating conjunction "that": then, that is the dependent clause and whatever remains is the main clause.

The main clause may be even shorter. In a sentence like: "Whoever arrives first is the winner," the subject "Whoever arrives first" is a nominal (dependent) clause, so only the predicate "is the winner" is the main clause, with an implicit or tacit subject "he/she". In a sentence like: "Say what you think," the direct object "what you think" is a content (dependent) clause and only the verb "Say" (with the tacit subject "you") is the main clause.

The main clause is actually "independent" in what we call the "deep structure" of the sentence, and sticking to the words that appear "on the surface" will not help you detect it, nor will the concept of a standalone, independent clause always help. The best thing for you to do is discard all the dependent clauses (not phrases), that is, smaller embedded sentences consisting of subject and predicate, and the main clause will be what remains.

Answer 2

Ah, the lies that we tell children.  A sentence is a complete thought.  A noun is a person, place or thing.  Elves make all the toys for Santa Clause.  As far as I can tell, none of those things are true.  Santa often gets his presents from the same stores where we shop.  A noun is a word that follows certain grammatical rules, no matter what it represents.  The basic unit of thought is not the phrase or clause or sentence, but the paragraph. 

When I call a clause independent, I mean that that clause is not a part of another clause.  When I call a clause dependent or subordinate, I mean that that clause is a part of another clause.  Your example sentence contains one independent clause and one subordinate clause.  The subordinate clause is obvious: "that can fly".  The independent clause is somewhat harder to describe. 

The clause "that can fly" is part of the noun phrase "the only animal that can fly".  That entire noun phrase, including the subordinate clause, is a predicate nominative subject complement.  It is a part of the independent clause.  The independent clause also contains the subject "this" and the verb "is". 
 

In other words, you're absolutely correct.  The words "this is the only mammal" do not represent the entire independent clause in your example sentence.  Instead, those are the words that belong only to the independent clause.  The words "that can fly" belong to both the independent clause and the subordinate clause. 

{ [This] | [is] / [the only mammal { [that] | [can fly] }] }

 

Finally, consider my first sentence: "Ah, the lies that we tell children."  If it is a sentence, then it is a sentence that has no independent clause.  The subordinate clause "that we tell children" modifies "lies" in the same way that "that can fly" modifies "animal".  Is it a correct sentence?  Obviously I think so, or I wouldn't have used it.  However, it is an incomplete sentence -- a sentence fragment. 

Fragments are not always errors. 

In this case I consider "that we tell children" to be a subordinate clause even though it is not a part of any other clause.  It's merely a part of a phrase.