How do we differentiate long vowels from short vowels in English

Unfortunately, there is no good rule - children struggle with this when learning to write. There are a few rules of thumb that hold in most cases, though:

• Diphthongs (ou, ie, ei, eu, ...) are long (accOUnt), unless they're unstressed and turn into a schwa (succOUr)
• Single vowels (a, e, i, o, u) are short before double consonants (AttAck)
• Single vowels are long if they are followed by a single consonant and then an 'e', all in the same syllable (sAve, mOle), and in -ing forms of such verbs (sAve -> sAving)
• Single vowels are short if they are followed by a single consonant in the same or next syllable, unless the above applies (cAt, At-lAs, nO-mEn-clA-ture)
• And loads of exceptions to all of these rules, and they vary across dialects and idiolects. The consonants 'r', 'w' and 'y' do funny things to vowels, too.

These might help a little:

http://en.wikipedia.org/wiki/Vowel_length#Short_and_long_vowels_in_English

http://improvingenglishspelling.blogspot.co.uk/2010/06/long-and-short-vowels.html

How do you know if a vowel in English is long or short? Here are five simple decoding strategies. Although these reading strategies apply to only one-syllable words, these strategies can be applied on the syllable-level in multi-syllabic words.

1. If there is one consonant after the vowel, the vowel will be short. (Examples: an, pet, big, hot, cup)
2. If two consonants follow the vowel, the vowel will be short. (Examples: ant, rent, mist, cost, crust)
3. If the vowel stands alone, the vowel is long. (Examples: me, hi, go)
4. If an E is at the end of the word and it is preceded by another vowel, the E at the end of the word is silent and the first vowel is long. Silent E makes the first vowel long. (Examples: name, Pete, fine, home, cute or dune)
5. If two vowels are adjacent (next to each other), the second vowel is silent, making the first vowel long. (Examples: main, dream, lied, road, fruit)

NOTE: It is important to clarify that the terms "long" vowel and "short" vowel do not indicte the length of the vowel, but rather the sound of the vowel. In linguistic contexts, the terms "long" and "short" are referred to as "tense" and "lax" vowels, respectively.

NOTE 2: Yes there are exceptions, but when you apply these rules on the syllable-level you will find that for the most part they work quite well.

The letters a, i, o, u (and sometimes e too) have two common short and long sounds, as in:

flat, flatten – inflate; hid, hidden – hide; dot, dotty – dote; tub, tubby – tube; (hem, hemmed – theme).

I have explained this short / long (or closed / open) vowel spelling system in http://improvingenglishspelling.blogspot.co.uk/2010/06/long-and-short-vowels.html like this: When a, e, i, o and u are followed by just one consonant, or several consonants and a vowel, they are 'closed' and are supposed to have a short sound, as in:

 am, ample, ten, tender, pin, pinked, drop, droplet, bun, bunting.

If a single consonant after a, e, i, o and u is followed by a vowel, they are supposed to be ‘open’ and long, as in:

 hale, halo; peter, period; fine, final; sole, solo; tube, tubular'. 

If a stressed vowel before a single consonant and another vowel is to stay short, it is supposed to be followed by a doubled consonant:

 attitude, petty, pinnacle, dotty, bunny. 

Hence: cut + er = cutter, prefer + ed = preferred, enter + ed = entered, cute + er = cuter.

Thousands of English words conform to this system. Unfortunately, there are also hundreds words which break the ‘closed /short' and 'open / long’ vowel spelling method in one or more of five different ways. ... My blog http://englishspellingproblems.blogspot.co.uk/2013/11/irregular-spellings-in-4217-common-words_7864.html shows all common words which do so.

The only English vowel spelling that has a completely regular pronunciation is ee, as in 'keep sleep deep'. All others have some exceptions - http://englishspellingproblems.blogspot.com/2009/12/reading-problems.htm. Beyond a very basic level, pupils have to learn to read and write English words one by one.