Themselves is a compound personal pronoun, representing multitude, in the third person, plural number, and masculine gender; according to Rule 11th, which says, "When the antecedent is a collective noun conveying the idea of plurality, the pronoun must agree with it in the plural number:" and is in the objective case, being governed by indulge; according to Rule 5th, which says, "A noun or a pronoun made the object of an active-transitive verb or participle, is governed by it in the objective case." Because the meaning is—indulge themselves;—i. e., the individuals of the multitude indulge themselves.
But is a disjunctive conjunction: and connects what precedes and what follows; according to Rule 22d, which says, "Conjunctions connect words, sentences, or parts of sentences." Because the meaning is—A young man, &c., but, ah! &c.
Ah is an interjection, indicating sorrow: and is used independently; according to Rule 24th, which says, "Interjections have no dependent construction; they are put absolute, either alone, or with other words." Because the meaning is—ah!—unconnected with the rest of the sentence.
Poor is a common adjective, of the positive degree, compared regularly, poor, poorer, poorest: and relates to nature; according to Rule 9th, which says, "Adjectives relate to nouns or pronouns." Because the meaning is—poor human nature.
Fallen is a participial adjective, compared (perhaps) by adverbs: and relates to nature; according to Rule 9th, which says, "Adjectives relate to nouns or pronouns." Because the meaning is—fallen nature.
Human is a common adjective, not compared: and relates to nature; according to Rule 9th, which says, "Adjectives relate to nouns or pronouns." Because the meaning is—human nature.
Nature is a common noun, of the second person, singular number, neuter gender, and nominative case: and is put absolute by direct address; according to Rule 8th, which says, "A noun or a pronoun is put absolute in the nominative, when its case depends on no other word." Because the meaning is—poor fallen human nature!—the noun being unconnected with any verb.
What is a pronominal adjective, not compared: and relates to conflicts; according to Rule 9th, which says, "Adjectives relate to nouns or pronouns." Because the meaning is—what conflicts.
Conflicts is a common noun, of the third person, plural number, neuter gender, and nominative case: and is the subject of are; according to Rule 2d, which says, "A noun or a pronoun which is the subject of a finite verb, must be in the nominative case." Because the meaning is—conflicts are.
Are is an irregular neuter verb, from be, was, being, been; found in the indicative mood, present tense, third person, and plural number: and agrees with its nominative conflicts; according to Rule 14th, which says, "Every finite verb must agree with its subject, or nominative, in person and number." Because the meaning is—conflicts are.