English is said to have one of the most difficult spelling systems in the world. There are several reasons for this. 1) The pronunciation of many words has changed over the years, but not their spellings. For example, the ‘k’ in ‘knife’ and ‘gh’ in ‘right’ were pronounced during the Middle English Period (12th to 15th century). 2) Vowels have also changed their pronunciation over the years. 3) The spellings of many words have changed but not their pronunciation. For instance, the word ‘doubt’ used to be pronounced as ‘doute’. The ‘b’ was later inserted because the word had its origin in the Latin word ‘dubitare’ which had ‘b’ in it.
The same groups of words are often pronounced differently in different words. For instance, ‘ough’ has six different pronunciations in bough, cough, thorough, thought, through and rough. Note also the different spellings of the ‘sh’ sound as in ocean, machine, special, nation, anxious etc.
There are, nevertheless, certain general rules for forming plurals, past tenses, adding suffixes and prefixes. Note that many of these rules have exceptions. It is therefore essential to pay as much attention to the exceptions as to the rules.
When a weak verb ends in a consonant preceded by a vowel, the final consonant is not doubled to form the past tense.
Budget, budgeted (NOT budgetted)
Offer, offered (NOT offerred)
Benefit, benefited (NOT benefitted)
Focus, focused (NOT focussed)
Bias, biased (NOT biassed)
The only exception to this rule is probably ‘worship’ which forms its past tense by doubling the final consonant ‘p’.
Worship, worshipped (NOT worshiped)
Note that if the accent falls on the last syllable, the final consonant is doubled.
So we have:
Occur, occurred (NOT occured)
Transfer, transferred (NOT transfered)
Begin, beginning (NOT begining)
If the final consonant is ‘l’, it is always doubled.
Travel, travelled (NOT traveled)
Level, levelled (NOT leveled)
Parallel is an exception to this rule.
Parallel, paralleled (NOT parallelled)
Note this rule is no longer strictly followed. Both ‘traveled’ and ‘leveled’ are now considered correct, especially in American English.