Run-on Sentences (also called “Fused Sentences”)

Do you recognize the following sentence as nonstandard?

Nonstandard

I forgot my keys this morning I don’t know quite how it happened.

A fused or run-on sentence occurs when two independent clauses are connected without any intervening punctuation. There are a variety of ways to correct it.

Standard Sentences

I forgot my keys this morning. I don’t know quite how it happened.

I forgot my keys this morning; I don’t know quite how it happened.

I forgot my keys this morning, but I don’t know quite how it happened.

I forgot my keys this morning even though I don’t know quite how it happened.

I forgot my keys this morning; however, I don’t know quite how it happened.

Please note that a run-on sentence is not just a particularly long sentence; long sentences—even sentences in excess of a hundred words—can follow grammatical rules entirely properly: when they do so, they are merely long sentences. In order to be considered a run-on, a sentence must have two independent clauses that are stuck together without any intervening punctuation.