About 90% of the time [f] is spelled one of these two ways.
3. Most of the time [f] is spelled <f> or <ff>.
4.f It is usually easy to know when to use <f> and <ff>. The <ff> is always there for good reasons. Most often it is due to assimilation or the VCC pattern, or it is between a short vowel and <le>. Less often it is due to twinning or simple addition.
With <ff> the VCC pattern rather than the VC# is usual at the end of words, as in stiff and staff rather than ∗stif or ∗staf. The only words that end with a single <f> following a short vowel are the French chef and clef and the English word if. So the only cases of [f] spelled <ff> due to twinning are in iffy, iffier, and iffiest.
In the following words, if the <ff> spelling is due to assimilation, twinning, or simple addition, analyze the word into prefix, base, and suffix to show where the <ff> spelling comes from. If the <ff> is due to the VCC pattern or is between a short vowel and <le>, just write ‘VCC’ or ‘<ffle>’ in the Analysis column. Remember that VCC rather than VC# is normal for [f] at the end of the word:
ad+ f + fect + ion
if + f + y
ob+ f + fer + ing
ex+ f + fect + ive
shelf + ful
in + dis+ f + fer + ent
ex+ f + fic + i + ent
sub+ f + fer + ed
if + f +y+ i + est
Item 4. The answer sheet gives full analyses for the words in this table although the students need only analyze the words enough to show the reason for the <ff>. The <i> insertion in efficient is, again, due to the demands of the pattern for the palatalized <c> spelling of [sh].
For more on the spelling of [f] see AES, pp. 377-84.