Commas represent a pause in a thought, and they do not require a full stop. They ask the reader to slow down and pause before beginning the next part of the sentence. The remainder of the sentence usually adds meaning to the first part of the sentence. The reader needs a chance to understand the first part before reading the next.
INCORRECT: Mrs. Smith my language teacher has crossed eyes and I never know where she is looking.
CORRECT: Mrs. Smith, my language teacher, has crossed eyes, and I never know where she is looking.
A semicolon in a sentence does not require a full stop, but it does require a pause that is longer than the pause required by a comma. When you see a semicolon that comes before an independent clause, you can expect that clause to say basically the same thing that the beginning of the sentence said. Remember that an independent clause is one that has a subject and a verb.
INCORRECT: Mrs. Smith’s eyes do not look straight at you they seem to look at each other.
CORRECT: Mrs. Smith’s eyes do not look straight at you; they seem to look at each other.
If a compound sentence does not use a conjunction between its two independent clauses, a semicolon is able to act like a period. In this way, a semicolon does make the same stop that is required by the use of a period. While semicolons can act like periods, writers should use this mechanical sparingly, as it can disrupt the flow of a passage if used too frequently.
INCORRECT: Mrs. Smith seems to have two eyes that do not match in size one is bigger than the other.
CORRECT: Mrs. Smith seems to have two eyes that do not match in size; one is bigger than the other.