SPELLING "ing"
1.If the verb ends in -e (silent)we leave out ewrite - writing
 Exception: be  we don't leave out e
be - being
 
If the verb ends in -ee
we don't leave out e
see - seeing
2.
If the last letters are CVC:
Consonant - stressed Vowel - Consonant
we double the last consonant
swim - swimming
begin - beginning
 
If the last letters are CVC:
Consonant - unstressed Vowel - Consonant
we don't double the last consonant
listen - listening
happen - happening
 
If the last letters are CVC:
Consonant - stressed Vowel - w/ x/ y
we don't double the last consonant
snow - snowing
say - saying
relax - relaxing
 If the last letters are CVC:
Consonant - Vowel - l
we double the last consonant (BrE)
travel - travelling (BrE)*
3. If the verb ends in -iewe change ie to y
lie - lying
die - dying 
4. In other caseswe just add ingwork - working
 
*NOTE! In American English (AmE) the letter "l" is not doubled: travel - traveling.
 
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BrE - British English
AmE - American English
CVC - consonant - vowel - consonant