One of the best ways to teach your child about complete sentences is to help them understand the basic parts of a complete sentence, a subject noun and a verb:
Jesus | wept
“Jesus wept” is a complete sentence on its own because it has those two parts.
Nouns name. This is one of the easiest concepts to teach in English because you simply can look around the room and name people, places, and things around you and your child. Next, name activities and ideas. These aren’t as easy, but you must teach these harder-to-name nouns to your child as well. Swimming (activity) and love (idea) are both nouns. These nouns are abstract because you can’t touch them. The subject noun is often doing the action of the verb.
Verbs prove to be much more complicated, so start with action verbs. Kids can understand these quickly because action verbs assert an action. (Go figure!) As in the above example, “Jesus wept,” knowing the definition of nouns and actions verbs makes it obvious who is being named (Jesus–noun) and what word asserts an action (wept–verb). Have kids stand, jump, sit, smack, clap, etc. Get them to show you can perform an action. This will help the concept stick in that beautiful child’s brain. (I’ll post more about linking and helping verbs in the future, but the link below covers those as well.)
These are the basic parts of a sentence. You only need two parts to build a sentence in our language. And by teaching your child to identify these two parts of speech, you are laying the foundation to help them understand how not to write sentence fragments; you are laying the foundation to writing complete thoughts; and you are building their language skills.
For more on nouns: https://www.grammar-monster.com/lessons/nouns.htm
For more on verbs: https://www.grammar-monster.com/lessons/verbs.htm