To be a complete sentence there must be a subject and a predicate.
A short sentence that has both is still a complete sentence.
Example – “He left.” This is a complete sentence. “He” is the subject and “left” is the predicate. A subject can be a simple one word noun like “he”. A predicate can be a simple one word verb like “left”.
A sentence that is missing either a subject (noun) or a predicate (verb) is not a sentence at all. It is a fragment.
Example – “Running up the hill to get away from the zombies.” There is no subject so this is a fragment. It is not a complete sentence. “We were running up the hill to get away from the zombies.” This is now a complete sentence with the subject “we”.
Example – “A very long winding road on the side of the hill.” There is no predicate so this is a fragment. “Winding” is an adjective here and not a verb. “A very long winding road on the side of the hill led nowhere.” This is now a complete sentence with the verb “led” and the predicate “led nowhere”.
Combining too many ideas or even words into a single sentence can create a confusing and grammatically incorrect run-on sentence. Failing to properly use commas, conjunctions (and, or, but, etc.), and semi-colons can also result in a run-on sentence.
As a general rule at this point in your writing development limit yourself to one conjunction (and, or, but) and a maximum of two commas per sentence. Breaking ideas into separate simpler sentences can make your writing easier to understand. And using commas to break up and organize sentences into logical sections also makes your writing more readable.
The most common run-on sentence is created when students combine two complete sentences into a single sentence without the correct punctuation.
Example – “He ran from the zombies and he hid in the cemetery.” This is a bad idea. There are likely to be more zombies in the cemetery. This is also a run-on sentence. “He ran from the zombies” is a complete sentence with a subject and a predicate. “He hid in the cemetery” is also a complete sentence with a subject and a predicate.
Combining 2 complete sentences into one sentence requires either a conjunction and a comma, or a semi-colon.
This run-on can be corrected in any of the following ways:
“He ran from the zombies. He hid in the cemetery.” Separate into 2 sentences.
“He ran from the zombies and hid in the cemetery.” Remove the second subject “he”.
“He ran from the zombies, and he hid in the cemetery.” Add a comma with the conjunction to combine 2 complete sentences.
“He ran from the zombies; he hid in the cemetery.” Use a semi-colon to combine 2 complete sentences.
“He ran from the zombies, he hid in the cemetery.” This is still a fragment! A comma or a conjunction alone is not strong enough to combine 2 complete sentences. You need both or a semi-colon.
Consistent Verb Tense
Changing verb tense within a sentence or even within a paper can create problems.
As a general rule, unless you have good reasons and a very good understanding of grammar, maintain the same verb tense throughout your paper. Keeping all verbs in past tense is the easiest way to avoid mistakes with verb tenses.
Example – “She attacked the zombie with her pen, and she keeps stabbing it in the forehead.” This is an effective way to kill zombies based on television shows. However this is NOT consistent use of verb tense. “Attacked” is the past tense of the verb. “Keeps stabbing” is present tense (present continuous). Maintain past tense for all verbs throughout the sentence. “She attacked the zombie with her pen, and she kept stabbing it in the forehead.” “Kept” is the past tense of “keeps” and maintains a consistent verb tense in the sentence.