I remember writing about that kind of thing on CELTA. Is that what you're doing now?
It looks like you're in a Spanish-speaking country. Native speakers of romance languages often have problems with the English /ɪ/ sound because the letter "i" is pronounced /i/ in their native language. That means you'd expect the pronunciation of the vowel in "-ing" to be problematic.
Do you have access to a copy of Learner English by Swan and Smith?