Future perfect continuous

[Unregistered]

To whom it may concern,

I would like some help in being able to identify the difference between the future continuous and the future perfect continuous. All the examples I have seen until now are not completely clear about which tense you should. For example:
By the time we get to Chicago this evening, we will have driven more than four hundred miles.
By the time she gets here, we will have been waiting for her for two hours.

Why does the one sentence use futre perfect continuous and the other one future continuous?

Sincerely yours,

Linda

 

[louhevly]

To whom it may concern,

I would like some help in being able to identify the difference between the future continuous and the future perfect continuous. All the examples I have seen until now are not completely clear about which tense you should. For example:
By the time we get to Chicago this evening, we will have driven more than four hundred miles.
By the time she gets here, we will have been waiting for her for two hours.

Why does the one sentence use futre perfect continuous and the other one future continuous?

Sincerely yours,

Linda

Basically they are synonymous; the difference between them is one of focus: "... we will have driven more than four hundred miles" focuses on the moment in which we are through driving, whereas "we will have been driving more than four hundred miles" focuses on the period of time we were driving.

In the same way "we will have been waiting for her for two hours" focuses on the time we have spent waiting for her, whereas "we will have waited for her for two hours" focuses on the moment in whcih we are no longer waiting.

HTH,
Lou