The spelling of words in English is difficult because they do not always adhere to established rules. Part of the reason for that is that English is an amalgam of Latin, Greek, French, Germanic, and other languages, all of which have their own idiosyncratic spelling conventions.
While most words in English are spelled phonetically, a great many are not. Their spellings simply must be memorized. There are, for instance, words with silent letters, such as lamb, climb, rhythm, thought, and many, many others. There are also words which seem to defy their own rules. For instance, why don’t cord and word rhyme?
Then there are homophones, words which sound alike, but have different spellings and different meanings, such as principal and principle, there and their, and sail and sale. Of course, there are words which sound different when pronounced, depending on what part of speech they manifesting, but which are spelled the same. Think about record, as in I like this record and We should record that song. There are words which have two correct spellings, such as theater and theatre, although, in American English theater is preferred.
How about the fact that an initial g followed by an e, i, or y usually sounds like j, as in gentle, ginger, and gymnasium, but it has a hard g sound in get? Why is that the case? In order to spell words such as those correctly, you have to understand that the spelling and pronunciation rule has to be ignored sometimes.
Then we have the problem involving irregular verbs. They totally defy spelling (and pronunciation) rules. Take, for example, eat. When that word is in the past tense form, it is not eated. It is ate. Why? The past tense of jump is jumped, so why is the past tense of eat not eated? For that matter, why is the past tense of read not readed? Why is it read? Why does it not change spelling from present to past tense?
Ok. As if that were not confusing enough, how about plural irregular nouns? The plural of boy is boys. That’s simple enough. Boy is a regular noun. But, what about woman? Why is its plural women? Why is more than one wolf spelled wolves? Why is parties the plural of party, but the plural of donkey is donkeys? Why is it that we simply add an s to donkey, but we have to change the y ending of party to ies? Why is it, for that matter, that some nouns are not differentiated at all when they are pluralized? Examples of that strange spelling rule are sheep, shrimp, and deer. The spelling does not change whether you are referring to one or more than one of those. It is not correct to refer to a flock of sheeps.
Why don’t invisible and advisable have the same suffix? Both ible and able sound the same, but different words require different suffixes. How about rambunctious and conscious? The suffixes of those two words sound the same. Why are they spelled differently?
Is there a rule for these spelling (and pronunciation) exceptions? Unfortunately, the answer is no. The only way in which to know the correct words to use in spoken and written English and to pronounce and spell them correctly is to memorize all of the unusual pronunciations and spellings and meanings. Careful reading is very helpful. Once you have seen a word used and spelled correctly in print, it is generally easier to remember how to use it and spell it correctly. A good comprehensive dictionary (not a pocket version, although a few are very good—I like The American Heritage Dictionary) is invaluable. It will include all of the information that you need in order to know how to use words correctly and pronounce and spell them properly. The spell check function on your computer will generally catch misspellings and lead you to corrections, but not always. It may not, for example, distinguish between their and there.
If you are concerned about your use of language and spelling, as I suggested, you should read and observe what you are reading. Another alternative is to submit your document to Papercheck.com for professional proofreading and editing.