Whether it is because of poor teaching or a natural love of stream of consciousness writing, many people, even the most intelligent and organized, write horrible run on sentences. They sound like a hysterical five year old telling his mother about his day: I walked in the class and Johnny threw a brick at me and I cried and the teacher came and asked me what was wrong and I hit Johnny and I was punished.
To set the record straight, a sentence is ONE idea structurally consisting of one SUBJECT, which may have more than one part, and one VERB, which may have more than one action as long as they all relate to the same subject.
In simple terms, the subject can be simple, as in Boys, or compound, as in Boys and girls. The verb can simple, as in laugh, or compound, as in laugh and cry. You cannot have two separate sets of subjects and verbs. Therefore, Boys and girls laugh and cry is grammatically correct while Boys laugh and girls cry is incorrect. Instead of the word and, you can use the conjunction while: Boys laugh while girls cry; Alternatively, you can use the word with and put the verb in the ing form: Boys laugh, with girls crying. Finally, you can write two sentences: Boys laugh. Girls cry. That is the stereotype of children’s reaction to violence.
So, the next time you write a sentence in an email or composition, make sure there are one subject and one verb. If not, please make the changes and make your teacher/editor much happier!