When the letters "I" and "E" occur together in words they are sometimes spelled "IE" and sometimes spelled "EI".

If the sound of the two letters together in a word is /i:/, we write IE:

relieve, field, piece (except after C - conceited, receive, deceive).

The exceptions to this rule include:

neither, seize, species, weird

If the sound of the two letters in a word is not /i:/, we usually write EI:

eight, their, neighbour

The exceptions to this rule include:

friend, patience