The long 'ee' phoneme can be spelt in many different ways.
beef cheap key chief deceive
It is often hard to know when to use 'ie' and when to use 'ei'.
For example, is it 'believe' or 'beleive'?
This rhyme can help:
'i before e except after c'.
For most words we use 'ie':
However, if it comes straight after the letter 'c' then we use 'ei':
As always, there are some exceptions (protein, seize, caffeine), but the rule generally holds true.