The teacher saw the students ____(play) on the playground at 5 pm yesterday.

The teacher saw the students 【playing】 on the playground at 5 pm yesterday.
see sb do sth 省略to的不定式,即动词原形做宾补,强调【动作全过程】
see sb doing sth v-ing做宾补,强调【动作正在进行】
see sb/sth done 过去分词做宾补,强调【宾语与该动作是被动关系】
这里,突出动作正在进行,因此用playing.再如：
The managers would like to 【see the plan carried out】when they come back.
The lost boy were last seen playing on the beach.
The witness 【 saw the defendant come over and enter】 that house through the window.
祝你开心如意!