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
Noun
any abstract or concrete entity; a person (police officer, Michael), place (coastline, London), thing (necktie, television), idea (happiness), or quality (bravery)
Pronoun
any substitute for a noun or noun phrase
Adjective
any qualifier of a noun
Verb
any action (walk), occurrence (happen), or state of being (be)
Adverb
any qualifier of an adjective, verb, clause, sentence, or other adverb
Preposition
any establisher of relation and syntactic context
Conjunction
any syntactic connector
Interjection
any emotional greeting (or "exclamation")
source: wiki