The two essential parts of the sentence are:
noun
adjective phrase
subject
predicate

The answers are subject and predicate. A sentence should have these two. If not, it won’t be a sentence. It could end up as a sentence fragment. A subject is always a noun, and it doesn’t have to have an adjective phrase for it to be considered as a sentence. The two essential parts of a sentence are always a subject and a predicate. It can be simple subject or compound subject, and simple predicate or compound predicate. 

Jdjdhdvdhdhsjsjsjsshjssj