Doubling consonants

When do you double a consonant when adding -ing or -ed?

First of all, consider this:

mat but mate
cut but cute
mit but mite
bit but bite
sit but site
dot but dote 
pet but Pete
pop but pope

As you can see, the effect of the final e is to lengthen the vowel that precedes it. It is as if the final e were not really a proper letter but a sort of delayed acute accent.

(I say "short" vowel  [lit, hit, mop, cap] and "long" vowel  [light, height, mope, cape], though the proper terms are, respectively, "open" and "closed" or "high" and "low" in reference to what your mouth and tongue are doing when pronouncing the vowel).

The problem is that when we drop the e before the suffixes -ing and -ed, how do we know how to pronounce the word? The solution is to double the consonant if you want to keep the preceding vowel short (open) and, conversely,  not to double the consonant if you want to keep it long (closed) (i.e. if  the original word ended with an e).

Thus we have mate-mating, but mat-matting (and matted). The double t tells you that the original sound was a short/open a.

Accordingly, cut becomes cutting, sit becomes sitting, dot becomes dotting or dotted, whereas dote becomes doting (what I hope your parents are with respect to you), and site becomes siting. Note becomes noting (no doubling of the t, therefore the o is long), and bite becomes biting.

This rule works for one-syllable words. With polysyllabic words, it becomes a little more difficult. Yet if you get this point clear, the rest follows quite easily.

Still, here's a teaser: Does combat becomes combatting or combating?
Well, for the British, the t needs to be doubled to keep the final a short, otherwise it is as if the original word were combate. For Americans, on the other hand,  given that word has two syllables, you do not have to apply the double-consonant rule: it's obvious the original word was not combate, as no such word exists. The answer? Both are acceptable.