“THE”: Before a vowel sound, THE is usually pronounced /ðі/ but /ðə/ before a
consonant sound. In very fast speech, native speakers sometimes use /ðə/ regardless of
a following vowel sound.
“A”:A is usually pronounced /æ/ but can be pronounced /ey/ when intentionally
stressed. In a sentence, A is often linked to the preceding sound and becomes an
“AN”: AN is pronounced /æn/ and links to the following vowel sound. AN can be
pronounced /ən/ if also linked to a preceding sound.
Sorry, I pasted this from my own notes, so the font & spacing is funky. Hope it helps though