Your sentence must have a subject and a predicate.
For example: The building collapsed. (building - subject; collapsed (predicate-verb)
You should also take into consideration the 8 parts of speech so that you will have a better guide.
Parts of Speech
any abstract or concrete entity; a person (police officer, Michael), place (coastline, London), thing (necktie, television), idea (happiness), or quality (bravery)
any substitute for a noun or noun phrase
any qualifier of a noun
any action (walk), occurrence (happen), or state of being (be)
any qualifier of an adjective, verb, clause, sentence, or other adverb
any establisher of relation and syntactic context
any syntactic connector
any emotional greeting (or "exclamation")