Look!Tim isthrowing a ball这句话为什么要加is?

Look!Tim isthrowing a ball这句话为什么要加is? Look!the dog is playing____ a ball. Look!The dolphin is playing ___ a ball who is the taller,Tim or Tom?这句话用加THE吗 The ball which is on the floor is mine.这句话对吗?为什么? why is Tim finding this trip exciting中的exciting为什么要加ing? 为什么这句话前read要加ing?reading books is a good for He is running这句话为什么run要加ning He is as a good boy as Tim.为什么要用as a good boy as? 英语不定冠词,急A horse is a useful animal.这句话中为什么要在useful前加冠词,anima Look,The ball is in the LOOK!THE BALL IS IN THE ______.