A E I O U and sometimes Y, but also W?
When Y and W Are in the Middle of a Syllable
What about in the middle of a syllable? In words like “rhythm,” Y represents a vowel, specifically the short I sound. However, in a word such as the name “Reynold,” it doesn’t make sense to say that Y represents a vowel. It’s the two-letter combination E-Y that represents a vowel.
As for W, it never occurs in the middle of a syllable, at least in native English words. There is the borrowed Welsh word “cwm,” [pronounced “coom”] spelled C-W-M, which refers to a kind of valley, but I’m only mentioning it because if I don’t, a commenter will. In this word, yes, W represents a vowel. Personally, I think this word is so rare as to be best ignored, but it’s allowed in Scrabble, so who am I to make a fuss?
When Y and W Are at the End of a Syllable
Last, let’s talk about Y and W at the end of a syllable. In one-syllable words such as “by” and “fly,” Y represents the vowel commonly known as long I. As an aside, long I is actually two vowels run together. Say it slowly enough, and you can hear that it consists of “ah” plus “ee.” The phonetic term for two vowels run together this way is diphthong, which sounds like an insult, and has actually been used as an insult by people who don’t know better.
Getting back to Y at the end of a syllable: In longer words such as “sorry” and “friendly,” it represents the vowel of long E (or maybe short I again—speakers vary). In words like “hey” or “day,” we run into complications. On the one hand, you could say that Y represents a vowel, because without it, we’d pronounce the words with different vowels. Instead of “hey,” we’d say “he,” and instead of “day,” we’d say “da.” But by that reasoning, you could also conclude that G and H in “fight” are vowels, too, because without them, the word would be “fit.” That way lies madness. The most sensible thing to say is that the letter combinations of E-Y and A-Y together represent the long A sound in these words.
What about the Y at the ends of words like “boy”? Well, “oy” is another diphthong, consisting of an O-like vowel followed by long E or short I. So you could say that Y represents one of those vowels. On the other hand, it might sound to you like the diphthong “oy” ends with the consonant “yuh,” so Y represents a consonant. Even phoneticians don’t all agree on this, so I recommend just saying that the letter combination O-Y represents the diphthong “oy,” and leaving it at that.