"I" Before "e"
Sometimes, I have made the mistake and seen others do it, it is the "i" before "e" in German, because the German language does not follow this rule and sometimes it is "e" before "i". My example is: frei
I believe GiroMaster is referring to an English spelling rule, that says (very*) generally when to use "ie" and when to use "ei" in a word; and that is namely:
"'I' before 'E', except after 'C'".
*This is about the only spelling rule anyone (I know) can name, however it probably has more exceptions than examples :P
A seemingly obvious point, but very important to remember - especially for native English speakers - as there are some similarities between the two languages, as they are related (both from the family of Germanic languages).
You wouldn't want to mix up "i" and "e" here:
„Bleib da, wo du bist! Beweg dich und ich schieß auf dich!“
Just going to link my other post here:
I'd say, you're assigning English values to the German vowels. In Modern English [iː] is spelled as ⟨ee⟩ and [aɪ̯] as ⟨i⟩ whilst German still spells [iː] as ⟨i⟩.
The short (stressed) German e is /ɛ/ and in the diphthong ei it corresponds to /a/. There are several ways to describe this difference, you could say /ɛ/ was lowered to /a/ in the diphthong or that the spelling is just a remnant of Middle High German. Regardless, the e before i stands for a more open sound gliding into a very high sound. ⟨ei⟩ has always been the spelling for a closing diphthong.
On the other hand i stands for /iː/ and /ɪ/. In ie, there is not a trace of any German e-sound after it. Neither /ɛ/ nor /eː/ and not even /ə/ (any more). That's why the e after i is silent. But it is not useless because it indicates the length of i. That's because German (and Dutch) used to have the diphthong ie /i͡ə/ which changed to /iː/. The spelling was reinterpreted as a spelling of a monophthong.
e+i is a diphtong, pronounced like "uy" in "to buy". Examples: Reise (trip), Wein (wine), Fleiß (diligence), Weise (manner), zeigen (show)
i+e is just a long German "I", pronounced like ee in "bee" Examples: Riese (giant), Wien (Vienna), fließen (flow), Wiese (meadow), Ziegen (goats).