I agree with Alaa it can be a little bit confusing but continue to read it over and over again. However, I gained a peripheral understanding of it this way:
CFO - Operating, business related transactions, buying, selling of goods, performance of services. Think day to day… operations.
CFF - Think Capital Structure which is primarily debt and equity related (only the debt and equity of the individual company, nothing outside of it).
CFI - Think INVESTING in real estate, PPE, anything that supports CFO but in the long-run.
Then there are exceptions to the rules that may differ between GAAP (almost always stricter) and IFRS - Interest and dividends paid/received. These can learned but become easier with an understanding of the above items.
Interest paid and received and Dividends received are CFO items under GAAP
Dividends paid are CFF items under GAAP (remember capital structure?)
Interest/Dividends paid - CFO or CFF under IFRS
Interest/Dividends received - CFO or CFI under IFRS
(Notice how IFRS is a tad bit flexible)
Hope this helps!