is walking是什么時態(tài)

　　例句：

　　They are playing basketball now.

　　現(xiàn)在，他們正在打籃球。

　　Listen， She is singing an English song.

　　聽，她正在唱英文歌。

　　Look at the picture. The children are flying kites in the park.

　　看這幅圖。那些孩子正在公園放風(fēng)箏。

　　We are making model planes these days.

　　這些天我們在做飛機(jī)模型。