When the word ends in a consonant
If the accent falls on the last syllable, the consonant is doubled to form the past tense.
So we have
Occur –> occurred
Transfer –> transferred
When the word ends in a short vowel + consonant, the final consonant is not usually doubled to form the past tense.
Therefore
Offer –> offered (NOT Offerred)
Budget –> budgeted
Short monosyllabic words always double their final consonant.
Examples are:
Shop –> shopping
Let –> letting
Cut –> cutting
‘ie’ and ‘ei’
The general rule is ‘i’ before ‘e’ except after ‘c’
Examples are:
Siege, believe, friends
But
Receive, deceive, ceiling (after ‘c’, we use ‘e’ before ‘I’)
There are however several exceptions to this rule. Examples are: reign, heir, seize, weird. As you can see, in all of these words, the letter ‘e’ goes before the letter ‘i’.
Dis and mis
Never double the ‘s’ of these prefixes. In some words, you may notice a second ‘s’, but remember that it is the first letter of the next syllable.
So we have
Dismiss (NOT Dissmiss)
Misplace
Misunderstand
Dispel
Se and Ce
Se and sy are usually verb endings and ce and cy are usually noun endings. So the following words are verbs: license, practise, advise, prophesy
And the following words are nouns: licence, practice, prophecy, advice
The word promise is an exception to this rule. Although it ends in –se, it is a noun.
Note that this rule does not hold good when verb and noun are not spelt alike.
Us and ous
Nouns end in ‘us’. Adjectives end in ‘ous’.
So we have:
Nouns: census, phosphorus, genius
Adjectives: jealous, unanimous, tremendous