Conjunctions like "and," "but," "or," and "so" are used to connect words, phrases, or clauses within a sentence. They cannot stand alone as the beginning of a sentence.
Here are some examples:
Incorrect: And the cat sat on the mat.
Correct: The cat sat on the mat.
Incorrect: But she went to the store.
Correct: She went to the store, but she didn't buy anything.
Incorrect: So he went home.
Correct: He went home, so he could rest.