In English, verbs are often changed by adding -ed (past tense) or -ing (for a sense of continuous action). These are known as verb inflections.
When adding -ed or -ing to a verb ending in e, the first spelling rule to remember is to ‘drop the e’:
bake baked baking
change changed changing
The second spelling rule to remember is to ‘double the consonant’ when the verb ends with a single vowel and consonant:
pat patted patting
step stepped stepping
but
peak peaked [because there are 2 vowels before the end]
camp camped [because there are 2 consonants at the end]
Exceptions to these rules
Verbs ending in -ee, -oe or -oo keep their 2 vowels before adding -ing or -ed:
agreeing/agreed decreeing/decreed canoeing/canoed tattooing/tattooed
Verbs ending in -ge generally drop the e before -ing, but retain the e if the meaning would otherwise be confused, or in a few uncommon verbs:
arranging changing exchanging impinging staging
but
singeing [which would otherwise be confused with singing]
Some words can be spelled either way:
bingeing/binging tingeing/tinging
and a few always retain the e:
whingeing
Verbs ending in -l, -r, -s or -t
For verbs ending in -l, the l is doubled before the -ing or -ed, in line with the rule to double the consonant:
compel compelling compelled
enrol enrolling enrolled
pedal pedalling pedalled
For verbs ending in -r, -s or -t, the letter is doubled when the word has 1 syllable, or has 2 syllables with the stress on the second:
Base word 1 syllable:
bar barring barred
scar scarred scarring
bus bussing bussed
bat batted batting
Stress on the second syllable:
defer deferred deferring
transfer transferring transferred
admit admitted admitting
regret regretted regretting
permit permitted permitting
But the t remains single if the stress is on the first syllable:
benefiting budgeting marketed targeted but formatting
For 2-syllable verbs with the stress on the first syllable, the letter is not doubled. For some -s endings, doubling is optional but less common in Australian usage:
differ differed differing
chorused chorusing
focus focused/focussed focusing/focussing
