the girl ( )(skate) on the river.为什么是现在进行时?

the girl ( )(skate) on the river.为什么是现在进行时? Mary often runs near the river.(改为现在进行时) Jihn,please swim in the river.改为现在进行时 The girl sitting on my left is my sister.中sit为什么要用进行时 The ski is thick enouge to skate.“skate”后面要加on吗?为什么? on the river The man sta on the safa （现在进行时） on the river?in the river? Are they (skate)on the lake? the ice is to skate on. 1.--Don't skate on the lake! skate on the ice什么意思?