1) Each sentence should have one main subject and one main verb, and expresses a complete idea.

Examples of sentences:

1. The boy threw a paper clip at the teacher.
2. The boy was sent to the principal's office.

Examples of incomplete ideas:

1. The boy who combed his hair forward in front of his eyes (needs a verb: what did the boy do?)
2. Rushed to get home before the television show was over (needs a subject: who rushed home?)

2) When you use two complete ideas (as independent clauses) in the same sentence, some punctuation rules apply. The main one is that they must be combined with a strong enough kind of punctuation mark.

The punctuation marks in ascending order of strength are:

Note that the comma is the first item on the above list. It is the weakest punctuation mark, and is not strong enough to punctuate a complete idea (an independent clause); the others are.

Examples:

NO: The boy threw a paper clip at the teacher, he was sent to the principal's office.

YES: The boy threw a paper clip at the teacher; he was sent to the principal's office.

YES: The boy threw a paper clip at the teacher: he was sent to the principal's office.

YES: The boy threw a paper clip at the teacher. He was sent to the principal's office.

YES: Because the boy threw a paper clip at the teacher, he was sent to the principal's office. Here the comma is okay because the parts around it are not complete thoughts:

1. Because the boy threw a paper clip at the teacher (not complete),
2. he was sent to the principal's office. (complete)