I'm confused about the tense of this sentence: "It is thought that more than 400 million people will have spoken English by the end of this decade". My teacher said that "will have spoken" wasn't correct and it must be "will speak". But I think "will have spoken" is correct or acceptable at least. Can someone explain it, please?

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@Fuongg

It's correct if what you mean is – 400 million people will have spoken English at least once. Like, they learned how to say one English sentence, and they said it. Once.

That doesn't make a lot of sense, does it? What you really mean is that these people will be speaking The language regularly. So you cannot say – will have spoken, as if everything is happening in the past. That is not what you mean. You mean that they are still speaking the language. So it should be – 400 million people WILL BE speaking English....