SVA = Subject + Verb + Adverbial
Elephants live here.
Elephants [SUBJECT] live [VERB] here. [ADVERBIAL]
In SVA sentences, the adverbial is required; without it, the sentence is grammatically incomplete:
Elephants live ??
Adverbs and adverbials are optional in the other 6 sentence patterns. You can include adverbials in any sentence you like, but the only sentence pattern that requires an adverbial is the SVA.