It might help you to cut up the word and associate the syllables with other words from the same root. In your example, -brev- means "short". We have the word "brief", which isn't so helpful because it has both i and e, but we also have "brevity" and the musical note called a "breve". There might be a mnemonic sentence you could use, such as "Never beLIEve a LIE" or "Beware of 'sepARATe' - it has A RAT!" See if those help as well.

There is no consistent rule unfortunately. "e", or "i" or "ie" (yield) or "ey" (key) or "ee" (feet) and other combinations can all make the "e" sound. This is one of the problems with the English language.......

OK

Maybe his pencil doesn't have spellcheck.

If it makes you feel better, native English speakers mess up "i"s and "e"s all the time. That's why God created spellcheck :).