A sentence cannot exist without a verb. The verb may be indicative, imperative, or subjunctive. The verb MUST be located before decisions can be made about the subject or the predicates.
(1) Is the verb SINGULAR? If the verb is singular, you cannot have a plural subject.
(2) Is the verb PLURAL? If the verb is plural, you cannot have a singular subject—though you can have two singular words,
which, when connected, form a plural idea and take a plural verb.
(3) Is the verb of the First Person? (I, we)
of the Second Person ? (you)
of the Third Person ? (he, she, it, they)
The subject might actually be expressed by a noun or a pronoun in the nominative case (singular or plural), or it might not. If there appears to be no subject-word, then consider supplying the correct English pronoun in accordance with the answer to question (3), above.
(1) If the verb is SINGULAR, the subject must be singular.
(2) If the verb is PLURAL, the subject must be plural.
(3) the VOCATIVE usually (and always, in the plural) looks like the nominative in form. It can be distinguished in modern books by being set off from the rest of the sentence by commas, and sometimes it may have the interjection 'O!" as a formal indicator (better not to try to translate 'O').
If the vocative noun is being addressed by the speaker of the sentence (either in the indicative or the imperative), then the vocative must agree in number (singular or plural) with the Second Person indicative or imperative of the verb:
'Girls (plural), give me the book.' [ = you (plural) give, imperative plural ]
'Girls (plural), the book is large.' [ Girls is a plural, but 'the book' and the main verb are singular. ]
'Girls (plural). you (plural) are beautiful.' [ 'Girls' is plural, the subject of the main verb and the main verb are plural. ]
(1) Is there a DIRECT OBJECT?
Some verbs never have a direct object (that is, they are 'intransitive verbs', they do not pass on the action of the verb from a subject to an object), so there is no point in looking for one.
(2) Is there an INDIRECT OBJECT? The Indirect Object is in the dative case (-ae, -is; -o, is; -i, ibus; etc)
Some verbs have both direct and indirect objects. The verb DO, DARE is worth special attention. You give something (accusative) to somebody (dative).
(3) Is there a preposition? If there is a preposition, it must have an object (otherwise, it's an adverb).
A noun or pronoun can almost always be paired off with its preposition. The object immediately or almost immediately follows the preposition, and this can help to eliminate nouns and pronouns as possibilities for direct or indirect objects.
There are two kinds of prepositional phrase, adjectival ('the man in the door') and adverbial ('The man walked into the street.')
John Paul Adams, CSUN