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grammCreating a run-on sentence instead of a full sentence is one of the most common grammatical errors made by writers, native speakers and non-native speakers alike.

A Run-on Sentence (sometimes called a "fused sentence") has at least two parts, either one of which can stand by itself (in other words, two independent clauses), but the two parts have been squashed together instead of being properly connected.

It is important to realize that the length of a sentence really has nothing to do with whether a sentence is a run-on or not; being a run-on is a structural flaw that can plague even a very short sentence:

The sun is high, put on some sunscreen.

When two independent clauses are connected by only a comma, they constitute a run-on sentence that is called a comma-splice. The example just above (about the sunscreen) is a comma-splice. When you use a comma to connect two independent clauses, it must be accompanied by a little conjunction (and, but, for, nor, yet, or, so).

The sun is high, so put on some sunscreen.

Run-on sentences happen typically under the following circumstances*:

When an independent clause gives an order or directive based on what was said in the prior independent clause:

This next chapter has a lot of difficult information in it, you should start studying right away.

We could put a full stop where that comma is and start a new sentence. A semicolon might also work there.

When two independent clauses are connected by a transitional expression (conjunctive adverb) such as however, moreover, nevertheless.

Mr. Chan has sent his four children to prestigious colleges, however, he has sacrificed his health working day and night in that dusty bakery.

Again, where that first comma appears, we could have used either a full stop — and started a new sentence — or a semicolon.

When the second of two independent clauses contains a pronoun that connects it to the first independent clause.

This computer doesn't make sense to me, it came without a manual.

Although these two clauses are quite brief, and the ideas are closely related, this is a run-on sentence. We need a full stop where that comma now stands.

Most of those computers in the Learning Assistance Centre are broken already, this proves my point about American computer manufacturers.

Again, two nicely related clauses, incorrectly connected — a run-on. Use a full stop to cure this sentence.