So, what does it mean and how is it built?
If we take off the suffix, we are left with what could be the base, <begin>. We could analyse this further, but I'm not going to right now because I'm going to use this word to show when letters double before adding a suffix, and when they do not.
So, we then have the word sum:
beginn + ing --> beginning
Why did that final <n> double?
Here's the convention, and it's actually very simple:
When you are adding a suffix that starts with a vowel and:
1. The stress is on the syllable directly before the suffix (as in beGIN)
2. There is one consonant at the end of the word with one vowel directly before it. (as in begIN)
Then double the last letter before adding the vowel suffix.
Here are some other examples of bases that double their last letter before a vowel suffix such as <-ing>:
Swim (one vowel, then one consonant at end. There is only one syllable so of course stress is on syllable closest to suffix)
Run (one vowel, then one consonant at end)
Hop
Spin
Zap
ForGET (Stress on syllable closest to suffix, one vowel then one consonant at end)
OMIT
But not:
Look (has two vowels preceeding the consonant at the end)
Walk (has two consonants)
RIVet (Stress is on first syllable, not the syllable closest to where the suffix will go)
Real Spelling has some fabulous flowcharts to lead you through the process of figuring out what to do when adding a suffix. I highly recommend them!
I hope you found this riveting!