Is This a Complete Sentence?
 Five Sentence-Testing Devices
As you may have noticed, the definition of a sentence as “a group of words that expresses a complete thought,” does not do the trick. The definition that some people use of a sentence as “a group of words having a subject and predicate” is the definition of a clause, not necessarily a sentence. (Tack a subordinating conjunction on a group of words having a subject and a predicate and you have a subordinate clause, not a sentence.) 
The reason why the “complete thought” notion of a sentence is largely ineffective is that it is abstract: Both words, complete and thought, denote abstractions, and abstractions are hard to learn.
To help students reach a better understanding of what a complete declarative sentence is, we recommend any one of the following, or any combination of the following. (Remember that we are talking about declarative sentences only.) These “sentence-testing devices” work because they manage to hit that invisible mechanism in the brain that recognizes complete sentences.
By the way, the notion of a complete sentence is important only in writing. When we speak, we are not obligated to use complete sentences, and we would be weird if we did so. It should be no surprise, then, that habitual readers (and those who have been read to a lot)  end up being far better at recognizing and creating complete sentences than those who read (and have been read to) only a little bit. Children and adults who engage in significant amounts of well-written reading material develop a natural feel for the sound of declarative sentences.
Device 1: The true/false test: 
It was Aristotle who postulated that a sentence is a proposition that can be stated to be either true or false. Any group of words that can be said to be either true or false is, in fact, a complete declarative sentence. 
Device 2: The “They believed that…” test: 
Try putting the words “They believed that…” in front of a group of words. Does it make sense? Does it hold up? Does it feel complete? If so then you have a complete declarative sentence. (If not, then you do not.)
Device 3: The “Tell me the news!” test: 
A sentence’s job is to tell you the news. Ask a group of words to tell you the news. If it does, then that group of words is a complete declarative sentence. (If it does not, then it is not.)
(Another way of accomplishing the same thing is to say “Guess what!” If the group of words “tells you what” when you say “Guess what!” then that group of words is a complete declarative sentence. (If it does not, then it is not.)
Device 4: The “tag question” test: 
A “tag question” is a little question that can be tacked on to the end of any declarative sentence to verify the truth of that sentence. For example, if the sentence is, “You live in New Jersey,” the tag question would be “don’t you?” If the sentence is “I have never lived in New Jersey,” the tag question would be “have I?” Native speakers can automatically generate tag questions. The mechanism for being able to do this—actually, pretty amazing—is installed in the language-processing part of the English speaker’s brain. But there’s no tag question for a group of words that does not constitute a complete declarative sentence.
Device 5: The bicycle test: 
Pamela Dykstra, author of An Easy Guide to Writing (Pearson), uses the two wheels of a bicycle as a metaphor for what a sentence is: a stable structure having two interdependent parts, with the subject (who or what is the sentence about?) as one wheel, and the predicate (what is the subject doing, having, or being?) as the other wheel. If you can “plot” your group of word on the two wheels accordingly, then you have a complete declarative sentence. (If you can’t, then you don’t.) 
Only one of these devices has to work for a given student to have a surefire way to know whether a group of words constitutes a complete sentences. Use whichever of these devices makes the most sense to you, as a teacher, and the one that you feel most comfortable teaching. Chances are good that the device you choose will work for most of your students. For the others, try the other devices, one at a time, and see what works!
Correcting (correctly punctuating) run-on sentences can happen only when students fully understand what a sentence (a.k.a. independent clause) is. Correcting run-on sentences is a matter of stitching clauses—independent and dependent—together by the proper use of punctuation and/or duly “licensed” words. Such words are conjunctions (coordinating and subordinating), relative pronouns, and conjunctive adverbs.