When does the verb double write and add ing emergency

When does the verb double write and add ing emergency

General situation: the following three conditions must be met at the same time
The last three letters of a word are: consonant + vowel + consonant
2. The syllables composed of these three letters must be stressed syllables (i.e. the apostrophe in the upper left corner of the phoneme in the phonetic alphabet; for example, the apostrophe in the upper left corner of B is stressed sign)
3. The vowels of the three letters must be short vowels (e.g. / I /, / E /...)
For example: stop, shop, swim, run, dig, hit
However, there are some other special cases. Although they do not fully comply with the above rules, they should also be written in pairs. Just remember them individually
Such as: prefer, refer (more common in middle school!)