Question about 'verb+ing'

[ashkan_tns]

Hey...I'd like to ask about when we add 'ing' to a verb,why sometimes the last letter of the verb must be repeated but sometimes no,as "begin+ing=beginning" but "wash+ing=washing"..... thanks if someone helps me......

 

[Wuisi]

When the last three letters fit this pattern: Consonant + Vowel + Consonant and the stress lays on the last syllable. - Visit->Visiting but Sit->SitTing. (There are some exceptions and differences between British and American English: travel, cancel, pedal...but they are not many)