This is a very important topic overlooked by many learners. Of course, perhaps even during the very first English lesson, everyone learns that “a” is pronounced as /ə/ (the “uh” sound) and that “the” is pronounced as /ðə/ (again with the “uh” sound). However, this is not always the case.
The first thing many beginners don’t know is that “the” before a vowel becomes /ði/ (thee), with the “ee” sound (as in “meet”, but usually slightly shorter) instead of /ə/. For example, “the orange” is pronounced as “thee orange”, “the apple” is pronounced as “thee apple”. The same happens also to “a”, which is then pronounced as “an”, but since it is also written as “an” (e.g. “an orange”, “an apple”), this rarely causes a problem.
Another issue is that the pronunciation of “the” and “a” changes when they are emphasized, but this is quite an extensive topic in its own right, so I write about it in a separate article.
‘U’ is a problem
Those who know that “the” is pronounced as “thee” before a vowel often struggle to pronounce it correctly before “u”, but the rule is actually very simple:
It doesn’t matter what letter follows after “the”; it only matters whether it is pronounced as a vowel. For example, “urinal” is pronounced as yoo-rin-l or yoo-raay-nl, i.e. it doesn’t begin with a vowel. Therefore, “the urinal” is pronounced with /ə/ (uh).
On the other hand, “umbrella” is pronounced as ahm-brel-uh, so there is a vowel at the beginning. Thus “the umbrella” would be pronounced as “thee umbrella”.
The same rule applies also to “a” and “an”:
So we would have: “a urinal” and “an umbrella”.
“The” and “a” before “H”
The same principle applies here. If the phoneme (sound) that follows is a vowel, we must use the “thee” and “an” form. Since “h” is not a vowel, we use “the” /ðə/ and “a” before it.
But there is a catch. There are many words in English in which ‘h’ at the beginning is silent, and if it is silent, we have to use “thee” and “an”, e.g. “an hour”. The link will take you to a list of such words.