After begin and start, the verbs understand and realize are only used in the infinitive.
- She began to understand what he really wanted. (Not: *...understanding...)
I have seen this note in this site, and I would like to ask if it is a general rule or not. I am asking this question to you because this is the first time I have ever faced like a statement.
Yes, as a general rule, that is correct.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.