English vocabulary has a variety of roots. Some words are from Germanic, some are from Latin (many by way of French), a few are from Greek. A few are loanwords from other languages. Partly because of this mongrel nature and because of the vicissitudes of history, there are no hard-and-fast rules for English spelling.
That said, there are a handful of guidelines that may apply to some commonly used words.
Don’t end a word with i, j, q, v, or u
Except for names (Lulu, Lorelei, Stepanov) and loanwords (Iraq, raj), no English word ends with one of these letters. If you find yourself ending a word with i, j, q, u, or v, it’s probably time to double-check with the dictionary.
C and G
C usually sounds like a k unless it appears before e, i, or y, in which case it sounds like an s. (Of course, ch usually follows a different set of rules.) G is usually hard (like the g in go) unless it appears before e, i, or y, in which case it usually sounds like a j.
There are some exceptions (especially for g – think of give or get*), but most of the time this rule will serve you well.
Choosing –ible versus –able
The suffixes –ible and –able usually indicate that a person or thing is capable or worthy of the specified action. It can be hard to know which words use which spelling, though. Quick, is it audible or audable? Adaptible or adaptable? If you’re unsure, you might appreciate this little tip.
Macmillan reports that most of the time, -able is added to a complete word (adaptable, doable, dependable). While –ible appears after an incomplete Latin root (audible, incredible, possible).
There are plenty of exceptions, like indubitable or collapsible, though. So, remember that this is just a guideline. Using eType is a better way to make sure you get the spelling right every time.
About that “i before e” thing…
You may have heard the following used as a mnemonic for English spelling: “I before e, except after c, or when sounded like a as in neighbor or weigh.” This is so off that there’s a nearly a whole Wikipedia page about the exceptions.
The last part – that you use e-i when the sound is a like weigh – is close enough to being universal that you could call it a rule. If a word has a long a sound, and you know an i and an e are somehow involved, it’s almost definitely spelled e-i (not i-e).
After c, “e before i” is a decent guideline – but only in words where the ei makes a long e sound, as in ceiling or receive. Exceptions are plural nouns (tendencies, democracies) and past-tense verbs and third-person singular present-tense verbs (fancied, fancies). C is generally followed by ie, though, when ie makes a sound other than (or in addition to) the long e (science, ancient, efficient, lacier, iciest).
The “i before e” idea, though, isn’t much good. Notable exceptions include either, neither, weird, caffeine, protein, foreign, seize, leisure – basically, too many to count.
There are very few consistent rules for English spelling. A few (like the first and second on this list) may be useful to memorize. But many (like the third and fourth) have too many exceptions to be worth the trouble. If you want to improve your spelling, memorizing tricks isn’t the way to do it. The best way to get spellings stuck in your head is to read a lot. Choose published sources, not online spaces where what you read isn’t edited and may contain errors. Other than that, a personal word list and eType may be your best friends.