מתי -ing?

1. בתחילת משפט, כנושא: Testing yourself is better than reading again.

2. אחרי מילת יחס (of, for, about, at, without, instead of, before, after): She is good at remembering names.

3. אחרי פעלים כמו enjoy, avoid, finish, keep, mind, suggest, practise, spend time: He keeps checking his phone.