Den Satz "Er fiel von Pferd." könnte man auch übersetzen als " He fell from the horse." Hier ist für mein ungeübtes Ohr schneller fassbar worum es da geht.
"off" mit zwei 'f' heiß immer etwas mit "weg von"/ "ab", Trennung von Dingen, die vorher zusammen waren. "of" mit einem 'f' zeigt eher eine Zugehörigkeit an. Um mal beiden in einen Satz zu packen:
I cut off the mane of the horse.
Beachten Sie auch den Unterschied in der Aussprache zwischen "off" und "of".
How can I communicate in writing the sound difference between "off" and "of"? If you look in an English dictionary, you'll see the pronunciation indicated by phonetic symbols. My point is to listen to native English-speakers, including the voices on Duolingo, and try to hear the difference. To my ear the two words sound very different. In any case, once you become attuned to the difference, you won't confuse them in the future. Listening is the key.
The vowel sound in "off" is the same as in "paw", and the "ff" has an f-sound. The vowel sound in "of" is the same as in "cut", and the "f" has a v-sound as in love. In fact, "of" is pronounced like "love" without the leading l-sound.
I have listened to many native speakers, live, on the radio, in films since childhood and never ever noticed a difference except of course that "off" gets more stress within a sentence than the unimportant "of" usually does.
But I just checked my dictionary and indeed find that "of" should be [ov] and "off" should be [of]. (No difference for the vowel though.) You live and learn! I'll keep a look out for that. (or would that be "keep an ear out"?)
Glad I could help. A good English dictionary will indicate a difference in the vowel sounds of "off" and "of". The American Heritage Dictionary actually lists three pronunciations for "off" and two for "of". In my comment I simply referred to the first-listed and most or more common pronunciation of each.