when从句的主句可以用一般过去时从句用过去进行时。when在这个句子中的汉语意思是“当……的时候”引导时间状语从句。例如:When we were planting trees ，a car ran by with great speed on the road.当我们正在植树的时候，一辆车飞速地在马路上驶过。
现在进行时的句子怎么写
一般现在时主语为单数第三人称时（she, he, it, 人名/机构名称/国家名/书名等等）,谓语动词为“动词原形末尾 + s”.
具体方法是：
1、多数为动词原形 + s：例如 read --- reads；cut --- cuts；kill --- kills
2、动词原形以“o” 结尾时 + es：例如 do --- does; go --- goes
3、动词原形以“ch”、“sh” ,“x” 结尾时+es ：例如 watch --- watches；wash --- washes；fix --- fixes；
4、动词原形以辅音字母+“y”结尾时,改“y” 为“i”,然后再加 es：例如 study --- studies.不过请注意,动词原形以元音字母加“y” 结尾时仍然只加“s”,例如 pay --- pays
5、某些动词的第三人称单数有固定形式,例如：be --- is; have --- has
另外,即使是“一般现在时”,情态动词是不区分人称和数的,例如：can,may 等用于一切人称的单数和复数.
20个现在进行时的句子
I'm doing my homework.
You're washing your coat.
He's watching TV.
They're waiting for the train.
What's going on there?
She's watering the flower.
I'm playing the piano.
You're having breakfast.
He's working on a project.
They're running.
He's swimming.
She's fishing.
He's talking to her.
We're argueing some problems.
They're drawing pictures.
Are you helping your friends?
The trees are being planted.
He's listening to the radio.
My father is repairing the car.
My mother is closing the door.