Why does the letter "a" sound when you start the word "apple" / a / and another sound when the letter "a" is an indefinite article ("a" big dog) / ə /

Which three parts of this passage from chapter 6 of emily brontë's wuthering heights show that hindley earnshaw did not care about the well-being of catherine and heathcliff after the death of mr. earnshaw? they both promised fair to grow up as rude as savages; the young master being entirely negligent how they behaved, and what they did, so they kept clear of him. he would not even have seen after their going to church on sundays, only joseph and the curate reprimanded his carelessness when they absented themselves; and that reminded him to order heathcliff a flogging, and catherine a fast from dinner or supper. but it was one of their chief amusements to run away to the moors in the morning and remain there all day, and the after punishment grew a mere thing to laugh at. the curate might set as many chapters as he for catherine to get by heart, and joseph might thrash heathcliff till his arm ached; they forgot everything the minute they were together again: at least the minute they had contrived some naughty plan of revenge; and many a time i’ve cried to myself to watch them growing more reckless daily, and i not daring to speak a syllable, for fear of losing the small power i still retained over the unfriended creatures. one sunday evening, it chanced that they were banished from the sitting-room, for making a noise, or a light offence of the kind; and when i went to call them to supper, i could discover them nowhere. we searched the house, above and below, and the yard and stables; they were invisible: and, at last, hindley in a passion told us to bolt the doors, and swore nobody should let them in that night. the household went to bed; and i, too, anxious to lie down, opened my lattice and put my head out to hearken, though it rained: determined to admit them in spite of the prohibition, should they return.