The British government has said this spelling rule is no longer worth teaching because there are so many exceptions to the rule. I think this is a mistake – a mistake to get rid of it and a mistake or misunderstanding of where the rule is supposed to be applied.
I was taught this rule is applied only specifically words with two (or more) syllables which have the long ‘ee’ sound such as ‘believe’, ‘reprieve’, ‘receive’ and ‘retrieve’.
According to the BNC nearly 2% of all words contain either the ‘ei’ or ‘ie’ combination their spelling. This means you will roughly come across this once in every 50 words in writing. Of these roughly two-thirds are ‘ie’ and the remaining one-third ‘ei’. A further one-fifteenth of ‘ei’ (0.04% of the entire English usage) is specifically ‘~cei~’.
This may seem like a small portion but experience will tell you that you come across this enough times to have to think about it when writing.
Let’s put it this way, this rule is catchy enough to stay with most people. It is a just matter of knowing when to apply it – that is, when coming across a long ‘e’ vowel sound usually after the second syllable.
The following is a list of words to which this spelling rule applies to: ACHIEVED ACHIEVER ACHIEVES AGGRIEVE BELIEVED BELIEVER BELIEVES GRIEVERS GRIEVING GRIEVOUS RELIEVED RELIEVER RELIEVES REPRIEVE RETRIEVE THIEVERY THIEVING THIEVISH ACHIEVE BELIEVE GRIEVED GRIEVER GRIEVES RELIEVE SIEVING THIEVED THIEVES GRIEVE SIEVED SIEVES THIEVE SIEVE and CEILINGS CONCEITS CONCEIVE DECEIVED DECEIVER DECEIVES PERCEIVE RECEIPTS RECEIVED RECEIVER RECEIVES CEILING CONCEIT DECEITS DECEIVE RECEIPT RECEIVE DECEIT.
In the real world one would come across one of these words in writing about once in a thousand words (or about four pages of writing). That is plenty to warrant the learner to remember this word … unless looking up a dictionary frequently is something they enjoy doing.
And remember: all the rest of the time the spelling can be worked out from the pronunciation.