Unlike many other languages (such as Spanish), English has no clear pronunciation rules. You may know the famous (or infamous) example that says we could spell "FISH" as "GHOTI" (GH as in "enough", O as in "women", and TI as in "station". As a result, English speakers have to memorize the spelling of words they learn from a very early age, hence the importance put in teaching it and even holding competitions such as the "Spelling Bee".
For Spanish speakers in the process of learning English, spelling often becomes a nightmare. This is both because they fail to understand its importance and because of the frequent "false friendship" among the Spanish and English pronunciation of certain letters, especially the vowels A, E, and I:
The English pronunciation of the letter A sounds almost exactly like the combination EI in Spanish, The English E sounds just like a Spanish I, and the I in English like the AI combination.
The frequent result of this is that we find that, in spite of the fact that most English learners memorize the alphabet at a very early stage of their learning process, even the most advanced ones get mixed up when spelling these vowels.
The easiest way to deal with this kind of spelling issue is mnemonics (you know, simple rules to help you remember somethhing). Here's a quick and dirty trick for Spanish speakers who get the vowels A, E, and I mixed up: everytime you hear or try to recall each of these vowel sounds, instead of thinking of the isolated letter, get used to thinking of it whithin a context that is easy to remember. For example, think of "ABC" (as opposed to only A), "EMAIL", and "iPhone" or "I am...". I guarantee this works. If you just make the effort of getting used to it, it will make your life much easier when spelling. Once you do it a few hundred times(Yes! That many! It is called mielinization, I'll tell you about it some other day), you will stop needing any tricks and it will all come naturally to you.
Good luck, and good S-P-E-L-L-I-N-G