The computer will not correct misuses of this type. Read, read, read, read. Read material of increasing difficulty. Research on students' vocabulary development and spelling skill indicates that both are related to quantity and quality of reading. The more you read, the more words you will learn, and the more easily you will be able to correctly spell words.
Punctuation ends sentences.
Punctuation connects sentences. Punctuation separates a series within a sentence. Punctuation separates modifiers in a sentence. Punctuation encloses or emphasizes interrupting material. Punctuation designates quotations from the rest of a sentence.