"I before e, except after c" Many school children are taught this rule. Then they quickly learn that it does not seem to work. What is not taught is that it usually works only for words where the "ie" is pronounced with a long e sound. (That is e as in eat.) So with that adjustment we can get rid of most of the exceptions, such as efficient, forfeit and science.
The "except after c" part usually only works when the "c" is immediately before the "ei". This helps explain why words like achieve and cookie are actually following the rule. So the long e may be spelled "ie" or "cei".
Long e examples: belief, believe, field, movie, niece, piece, thief, yield
Long e after c examples: ceiling, conceit, deceit, deceive, perceive, receipt, receive
The rule was created to try to explain the spelling of words that had already been taken from other languages. English spelling is difficult. Unlike English, most (alphabetic) languages follow fairly standard spelling rules. English has taken words from many other languages and the spelling often reflects the original languages’ rules. So there are some exceptions to English spelling rules:
The word seize appeared in English in the mid-13th century from the Old French word seisir "to take possession of" (Which may have in turn been from the Latin word sacire.)
The word caffeine is from the German word kaffein, coined1 by chemist Dr. Runge from the German word kaffee, meaning coffee. (or from the French word caféine)
The word albeit is not a foreign word, but is a compound word (late 14th century). It is a contraction of al – be – it "al(though) it be (that)."
The word protein is from the French word protéine, coined1 in 1838 by Dutch chemist Gerhard Johan Mulder. (perhaps from the Greek proteios, meaning "the first quality")
The word species is from the Latin word species meaning kind or type. It is related to the word specere meaning to look at or to see. The use in biology is only about 400 years old.
1 To invent a new word and have it accepted by people is to coin a word.
ei / ie forms that are not part of the rule
Words with "ei" often have a long a sound (That is a as in cake.) In these words the long a sound is generally spelled "eigh", not "ei".
Long a examples: neighbour, weigh, weight, eight
When "ie" makes a long i sound (That is i as in pie.), it is usually at the end of a word.
Long i examples: lie, pie, tie, die
There are some examples for the long i sound where "ie" is not at the end of the word:
The word fried has a long i, but this is different because the root word is fry. Words ending with y also often have a long i or long e sound. Verbs that end in y usually change y to ie before adding s.
Other words like quiet and diet do not have their "ie" at the end of the word. The word quiet comes from the Latin quies (generative case: quietis). The word diet from the Latin dieta (The "ta" is an inflection).
There are two correct pronunciations of either and neither starting with either a long i or long e sound. The long i pronunciation is the most common, in British English, and it fits the rule.
Words where i and e are together, but pronounced separately are not part of this rule either. This is usually found in the –ier suffix. For example: carrier, fancier, foggier and occupier. (Words such as comparatives and superlatives often are of this form.)