2.They were dancing at nine o'clock last night.
2.在there be结构中。如:There is a boy swimming in the river.
3.在have fun/problems结构中。如:We have fun learning English this term.
They had problems getting to the top of the mountain.
4.在介词后面。如:Thanks for helping me. Are you good at playing basketball?
What /How about doing sth? 做某事怎么样? I am interested in playing football.
5.在以下结构中
1.
2. enjoy doing sth 喜欢做某事;
3. finish doing sth; 完成做某事;
4. feel like doing sth 想要做某事;
5. stop doing sth 停止做某事(原来的事)
6. forget doing sth 忘记做过某事;
7. go on doing sth 继续做某事(原来的事)