Doubling final consonants
Ken Lee from Malaysia writes:
  Some verbs double the final consonant and some do not, e.g. slide - sliding does not, but chop - chopping does. Why is it so and how to know when and with what verbs to add just -ing or to double the consonant before adding -ing?
Roger Woodham replies:
slide - sliding / smile - smiling / hope - hoping
The clue in these examples, Ken, is that the consonant does not come right at the end of the verb. All of these verbs, and many more in infinitive form, end in -e. As you can see, what happens in all these cases is that we drop the -e before we add -ing:
I am hoping to see her on Thursday. I hope she's feeling better by then.
Keep smiling! If you can smile in spite of your illness, you'll win through.
Did you see him slide on the ice? He was sliding about all over the place.
see - seeing / agree - agreeing
Note that if the verb ends in -ee, the final -e is not dropped when -ing is added:
I could see you standing there on the thin ice. Seeing you standing there made me nervous.
Agreeing a date for our March meeting proved impossible. We had to agree not to meet in March.
slide - sliding / slip - slipping / sleep - sleeping
If we slip (on the ice, i.e. accidentally) rather than slide on it, the final consonant is doubled when -ing is added. This is because the consonant comes at the end of the word and is preceded by only one vowel letter and a short vowel sound. If there were two vowel letters and a longer vowel sound, as with sleep/sleeping, the consonant would not be doubled:
I'm just slipping out for a coffee. Do you want some? ~ Don't bother. I'm going to slip out myself for some fresh air.
She was sleeping on the floor by the coffee machine. ~ 'You can't go to sleep there,' I said. 'Have some coffee. It'll wake you up'.
b > bb / d > dd / g > gg / l > ll / m > mm / n > nn / p > pp / r > rr / t > tt
These are the final consonants that are doubled when -ing is added. And not only when -ing is added. They are doubled before any ending that begins with a vowel, e.g the past simple or past participle ending -ed and the comparative and superlative endings -er and -est. Compare the following:
I grabbed his shirt to slow him down. 'Don't grab my shirt!' he shouted.
He was sad because Arsenal had lost, sadder than I'd ever seen him before.
Bergkamp doesn't like travelling by air. He prefers to travel by train.
If you want to stay slim or be slimmer, just have some salad for lunch.
I think you shouldn't ban smoking in pubs, but banned it soon will be.
He was gulping, not sipping his wine. 'You should sip wine', I said.
My wife prefers red wine, but I've always preferred white.
It's going to be hot today. It may prove to be the hottest day of the year.
pack - packing, climb - climbing
Note that verbs ending with double consonants, e.g. pack - packing, climb - climbing, are not affected. Neither are longer verbs that end in unstressed syllables, e.g. visit - visiting, offer - offering. (Note travel - travelling is an exception here.)
panic - panicking
Verbs ending in -c change to -ck before -ing, etc, is added:
It's important not to panic if you lose your way. Panicking will only make matters worse.
write - writing - written / bite - biting - bitten
Note the vowel change from long to short and the doubling of the consonant in the past participle form of these two verbs:
I've been bitten by your dog! ~ That's impossible. My dog never bites anyone.
I'm writing to say I'm sorry about the dog bite. I should have written earlier.