I know, a lot of confusion about speeling english words exist, because the vowel sounds A, E, I, and Y are so similar. This is not the case with the romance languages-each vowel has distinct and easily remembered sounds. Plus, the english vowels have long and short sounds-how did this come about?
But not about the spelling of the word “spelling”. 
Three words: Great Vowel Shift.
If you listen to someone reading Middle English, the vowels tend to sound much more like their continental counterparts.
I am not a linguist, but responses have been slow, so here’s something to read. Hopefully someone more knowledgeable will stop by. I find this topic fascinating.
I’m not really sure why we still teach kids about “long” and “short” vowels. When I was in kindergarten, I learned that there were 5 vowels, a e i o u (and sometimes y and w!). Each vowel had a “long” and “short” version. See wiki. Granted we all already knew how to speak English. I think they were trying to teach us how to “sound out” new words (or “add letters” as one of my younger cousins called it.)
a, bait (long), bat (short)
e, beet, bet
i, bite, bit
o, boat, bot
u, poop, pup
This organization is somewhat bogus. Sure, it has some historical relevance, as John Mace explains, but it’s not how linguists (AFAIK) describe vowels, and it isn’t really going to help much when trying to learn how to pronounce words in another language. I’m going to try to paste IPA in here. I don’t know if it will show up for everyone. These 10 vowels are represented in IPA as:
ei, æ
iː, ɛ
ai, ɪ
oʊ, ɒ
uː, ʊ
Notice that the “long” vowels are all diphthongs.
I haven’t really answered your question. Why is the speeling all screwed up? In Spanish, at least the Mexican version I learned, you pretty much have 5 written letters (a e i o u) representing 5 vowels (/a/ /e/ /i/ /o/ /u/). Diphthongs are represented as rather obvious combinations of these written letters (wikilink). Why can’t English be so simple? Part of the problem is that we have a few more sounds than we have letters. The letter “I” often represents /iː/ (beet), /ɪ/ (bit), and the diphthong /ai/ (bite). Some languages get away with this by using various diacritical marks on the letters.
An other major problem is that the language and pronunciation have changed a lot. All languages change (despite the efforts of some), so I don’t know how this compares to other languages. Middle English is very difficult for me to figure out, although it can be done fairly easily with some minimal training. Anyone know how 14th-century Spanish compares to modern? Old English is a foreign language as far as I’m concerned. This wikipedia search shows several articles about vowel changes. Vowel change happens in any language, but also English gets the extra special screwiness of having early Brythonic influence, several Germanic invasions after getting established, and a big French injection in 1066.
To top things off, spellings are often conserved even though the word sounds completely different. Sometimes spellings and pronunciations change “sympathetically”. It’s been a while since I took historical and comparative linguistics, so I can’t provide examples.
And now that I’ve written all this, I see that there is a nice article on English orthography which might be the best answer to your question. Note that it’s not just vowels that can cause problems. See, “Though the tough cough and hiccough plough him through” quoted in that same article.
The long and short u are:
/ju/ … and … /ʌ/
as in puke and pup
The /uː/ and /ʊ/ are the long and short oo, as in pool and pull
But, yeah, the “long and short” system is pretty bogus.
It’s hard to answer your questions without knowing what sounds specifically you view as being similar. If you hear the vowels in pat, pet, and pit as being similar then it’s probably because the sounds in question don’t exist as separate phonemes in your native accent or language. To native English speakers (of most accents), they don’t sound similar.
A key to understanding English is that the letters used to represent vowels are not themselves vowels. There are at least a dozen vowel sounds in English (depending on your specific accent; some might have in the range of 20) and several more diphthongs. All these sounds must be represented by some combination of a, e, i, o, u, y, and w, and sometimes some other letters (as in eigh).
Think of this series, each using a different vowel (maybe):
peet, pit, pate, pet, pat, paht, poot, put, pote, pawt, putt, pot, part, pert, pute, pite, peer, pair, parry, Perry, pore
The spelling of English words was at some point standardized, not all with reference to a single accent either. So some words’ spellings are with reference to one 16th century accent and some words are spelled according to another’s. And words that enter English from other languages often preserve their spellings. Accents have continued to change, but not spellings.
Many European languages have “long and short” vowel sounds system. English is notable in that this categorization makes no sense.
Part of what the OP may be asking is about the schwa, rendered by linguists as an upside-down [e] that (loosely) makes the sound “uh.” It is represented in English by any number of different vowels, usually chosen on a historical basis. Latin speakers sounded the difference between the proto-forms of -ance and -ence; we do not, because we tend to reduce unstressed vowels to a schwa.
Allow me to gesture vaguely in the direction of an interactive IPA vowel chart, while we’re on the topic.
:eek: I have been waiting my whole life for that…
Silly me for not asking on the 'dope. Thank you for sharing.
It’s my pleasure. ^^ You’ve probably already found it, but the main page on that site has links to interactive versions of the rest of the main chart.
The really cool thing, though, is that the vowel chart is mildly schematic: if you superimpose the vowel quadrilateral over an average adult’s mouth, with “i” at the top-front of the the oral cavity and “ɒ” down and towards the back, the placement of each phonetic symbol within the quadrilateral roughly corresponds to the point of greatest stricture formed between the tongue and the top of the mouth when you articulate that vowel.
(I never get tired of seeing a room full of first-year phonetics students all sticking their fingers in their mouth during the midterm to check their tongue position against one of their answers.)