To write a complete sentence, applying proper rule of English grammar, you must use several different items. Make sure that your sentences have all of these elements:
Subject–verb pair: The verb is a "doing' or a "being' word. Someone or something has to do the action or exist in the state of being. That is the subject. Every complete sentences needs at least one subject–verb pair.
End punctuation: Every sentence ends with a punctuation mark. A statement ends with a full stop, a question with a question mark, and a strong statement with an exclamation mark.
Complete thought: The sentence must include one complete idea.