Spelling Words With -ing, -ed, and -er endings

Why does "begin" become "beginning" and "beginner"? Here is the rule for doubling a letter:

If all three of the following are true, then double the final letter before adding -ing, -ed, or -er.

1. The word ends in a single consonant.

2. The single consonant is preceded by a single vowel (a,e,i,o,u)

3. The accent (stress) is on the last syllable.

--Think about "begin". Do we stress the "be" or the "gin"? We stress the "gin": we say be-GIN, no BE-gin. So we double the final letter before adding -ing, -ed, or -er.

Two examples:
keep: number 1 and 3 are true, but #2 is not: there are two vowels before the single consonant. So we write "keeping".

wed: All three are true (ends in "d," one vowel before "d", and stress on the last (only) syllable), so we write "wedding".