Not sure that my explanation makes sense

be 是系动词， 表状态（是或不是, 不存在‘正在是’）
Dog has been the most friendly animal to human since human started to domesticate wild animals.


walk 作为动词， 可以讲‘正在走’
Human have been walking using only two rear limbs since they were no longer apes.(一直用两条腿走， 现在亦如此）

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• Thanks. -肖庄- ♀ (0 bytes) () 02/03/2015 postreply 20:22:21