pat/patted, ram/rammed, bit/bitten ..."
The reason for this is to prevent a change of sound which would otherwise occur when you place and /i/ or /e/ after a monsyllabic word ending in a vowel and consonant - " bit/bite, pat/pate, run/rune" etc.
It's rare to double the syllable in a multisyllabic word when the stress is not on the last syllable (as in your examples, open and listen).
However in BrE, words ending in /l/ like "travel, unravel, channel" change to "travelled/travelling/channelled, unravelled/unravelling/channelling", while in AmE, they don't double the letter.
That's a few hints that I can think of at the moment.
PS: Have you tried here:
- For Teachers