In the sentence "What a dangerous snake it is!", the subject is "it", the auxiliary verb is "is", the main verb is "is", and there is no object. The sentence is an example of a predicate nominative sentence, where the subject is linked to a noun or adjective by a linking verb. In this case, "it" is the subject and "a dangerous snake" is the predicate nominative, which is an adjective phrase that describes the subject.