百问百答：in the playground还是on the p

in the playground/on the playground都可以

in用于三维运动场

on用于二维运动场

Whether to use in or on is a matter of Dimensionality. Since playground can be thought of either as a 2-dimensional bounded ground surface (hence requiring on), or as a 3-dimensional outdoor space (hence requiring in), both on the playground and in the playground are correct and both are common. Their metaphor themes are coherent -- i.e, if one is on the bounded 2-D play ground, then one is also in the 3-D space it defines, and vice versa.

He is playing in the playground

...means that he is inside a defined area, set aside for play, referred to as the playground, possibly with the implication that this playground is a standalone area. It would normally be used when the speaker is outside the playground. He is playing on the playground ...is a more context-sensitive. It could mean: That 'he' is on the designated playground area attached to a school or other facility That the play area is divided into a grass area and a paved or padded area, and 'he' is playing on the prepared area That there is a climbing set or other play equipment set up in the play area, and he is playing on that equipment (commonly colloquially called a playground). This would also more often be used if the speaker is inside the area being referred to, to specify a more precise location within the school/play area.