Scientific Writing

A sentence fragment occurs when a sentence is missing one of the key parts necessary for it to be a complete sentence. Typically, a fragment lacks a subject-verb relationship.

For example, these are a few sentence fragments:

During the night

Notice that this clause locates something in time/space, but doesn't tell us what is happening.

Some of the students working in the lab.

This clause identifies a subject, but doesn't explain what the subject is doing. Remember, for an -ing verb to be an action, it must be immediately preceded by another verb (e.g., we are working).

Even though he worked tirelessly

This fragment actually contains a subject-verb relationship (he worked), but the phrase "even though" makes it clear that this is a subordinate clause*, and therefore it needs another clause to complete the sentence.

*A subordinate clause almost always will begin with a prepositional phrase (e.g., "even though", "despite", "although")

Now that we have looked at these sentences as fragments, let's take a look at how they would look completed

During the night, our cat ran away.

We now know what occurred during the time specified by "During the night".

Some of the students working in the lab found extra supplies.

Now our sentence has a verb to match our subject ("the students... found").

Even though he worked tirelessly, he failed to complete his paper.

Notice that now we have added an independent clause (a clause that can stand as a sentence on its own) to this sentence. Now we know what the initial clause ("Even though...") is subordinate to. 

Contrary to popular belief, a run-on sentence is not a sentence that simply goes on too long. A run-on sentence is a sentence where two different ideas are in a sentence, but there is no punctuation or conjunction to separate them.

The following is an example of a run-on sentence:

I went to the store I bought some milk.

Notice that there are two different clauses in this sentence: "I went to the store" and "I bought some milk." We need to add something between these two clauses to make clearer where one ends and the other begins.

In this sentence, we have two options for correcting the run-on:

1. I went to the store. I bought some milk
2. I went to the store, and I bought some milk.

We can choose to separate these two clauses into two separate sentences, or we can add a comma and a conjunction (in this case, "and").