A run-on sentence is a problem in writing. It is a sentence that
is too long. Often run-on sentences contain:
•        More than one idea
•        More than one subject

Example:

I went to the store to buy milk and the milk was very cheap
because it was on sale.

I and the milk are both subjects. There are two ideas in this
sentence as well.

Take a look at other examples of run on sentences. The
multiple subjects are underlined. Remember, these are all
mistakes. You want to avoid run on sentences in your writing.

The boy is very happy, his sister is not.
Mr. Lewis and Mr. Jenkins are neighbours, they have known
each other for 5 years.

The general rule to avoid run on sentences is as follows:

       A sentence has one subject and one main idea.

Conjunctions are often used improperly to create run-on
sentences so be aware of words like and, but, so, and then.

Many people use commas incorrectly. The result is a run on
sentence called a comma splice. Look at some examples of
comma splices to see the mistake.

Please do not use your dictionary, they are not allowed on
the test.
The man has never visited France, his brother has.

Solutions to Run-On Sentences

Use a period to make two sentences

Hank loves baseball and he plays every Sunday.
Hank loves baseball. He plays every Sunday.

Separate with a semicolon (;)

Hank loves baseball and he plays every Sunday.
Hank loves baseball; he plays every Sunday.

Separate with a comma and add but, or, yet, so, for, and, nor

Hank loves baseball and he plays every Sunday.
Hank loves baseball, and he plays every day.
Hank loves baseball, but he never plays.

Practice Run-on Sentences with this test:

Run on Sentences Test

Run-on Sentences Test 2