Your question and examples are rather confused, but this is the rule: You double the consonant if you have the combination CVC (consonant/vowel/consonant) as the last three letters of the word. This applies to single-syllable words (such as 'rip') and to polysyllabic words where the last syllable bears the stress ( such as 'refer'). rip --> ripped trim --> trimmed hug --> hugged knit --> knitted tip --> tipped and refer --> referred permit --> permitted These are the only cases where the consonant is doubled. The past tense of 'knife' is 'knifed'. This is similar to hope' --> hoped, style --> styled, and so on - if you have an 'e' at the end of the word, you just add a 'd'. One grey area is verbs ending in 'l' which are spelt - or 'spelled' :) - with a single 'l' in US English (traveled), but a double 'l' in British English (travelled) even though the last syllable isn't stressed.

Actually, prefer, permit, and refer do get double consonants when put into past tense. (preferred, permitted, referred) In Britain, I believe travel gets a double consonant as well. I don't know why knife doesn't though... perhaps it has something to do with the long 'i' sound and silent 'e.' Or the fact that 'kinfe' ends in a vowel and the others end in a consonant. I hope someone knows!

Either it is too early morning for me, or you have a mistake there. It should be "knifed". And "traveled" is also OK. "travelled" is mostly British way of saying it.