For by story I mean this: the synthesis (putting-together) of the actions; and by characters I mean that according to which we say the ones acting are of a certain sort, and by thought I mean that in which, while speaking, they demonstrate something or declare their opinion. Necessarily, therefore, there are six parts of all tragedy, according to which tragedy is of a certain sort. And these are story, characters, thought, appearances, diction, and poetic song. For two parts are those which they imitate , one part is how they imitate, and three parts are what they imitate; and there is nothing in addition to these. By these, therefore, not a few have been used to say that [something is a tragedy] by these forms. For even appearance has all [of tragedy] , and in a similar way, character, story, diction, song, and thought.
But the greatest of these is the synthesis (putting-together) of the actions. For tragedy is an imitation not of human beings but of actions and of life. Both happiness and wretchedness are in action and the goal is some action, not a quality. On the one hand, they [human beings?] are of a certain sort according to their characters, but actions [are of a certain sort] according to happiness or the opposite. So they do not act so as to imitate the characters, but they include characters on account of the actions. So as the actions and the story are the goal (telos) of tragedy, the goal is the greatest of all of them. Moreover, without action tragedy could never come to be, but without characters it could come to be. For the tragedies of most of the latest poets are without characters, and generally many poets are such as this: like the painter Zeuxis is to Polygnotus. For Polygnotus is a good painter of character, but the paintings of Zeuxis do not have character. Yet if someone lines up a row of declarations about character, having made them well concerning speech and thought, he does not make that which was the function of the tragedy, but a tragedy, having supplied these things, is lacking less than one having both a story and a synthesis (putting-together) of actions. In addition to these, the greatest thing of tragedy is the parts of the story that lead the soul: reversal and recognition. A sign of this is that those who attempt to make poetry are first able, like nearly all the first poets, to accurately put together the speech and the characters more than the actions. Story, therefore, is the ruling principle and like the soul of tragedy, and the characters are second. (And it is pretty much the same with painting, for if someone smears on the most beautiful colors in heaps it would not give as much pleasure as an outline of an image in black and white.)
 I'm not sure what the 3rd plural "they" is referring to.
 The text of this sentence is, to use a scholarly phrase, all jacked up (from the Latin, allus jactus upus). The basic sense seems to be that sometimes tragedy is identified with a particular one of its six parts.
 This phrase = a.j.u.
 Christ's text includes an extra kakodaimonias ("wretchedness") after biou ("life"). A variant excises it. I followed the variant because I wouldn't know what to do with an extra kakodaimonias.
 From LSJ, peripeteia = "sudden reversal of circumstances on which the plot in a Tragedy hinges, such as Oedipus' discovery of his parentage"; this passage from Poetics is cited. And anagnwrisis = "recognition," again a technical term in tragedy, and the Poetics is cited (1452a29 and 1454b19).