Sunny's question of March 4, 2000:

'I'll be leaving at seven.'

Why are there double expressions of the future tense, 'will' and 'be leaving'?

This isn't actually a double expression--it's an aspect of the future called the future continuous tense.

Examples:

I'm studying now. (present continuous)

I will be studying at this time tomorrow. (future continuous)

　

Note that we can express the future using the present continuous tense ("I am leaving at seven."), but the meaning is slightly different. If we use the future continuous tense, we are literally saying,"At that time this is what I will be doing." The future continuous sounds a little more casual than the present continuous--it sounds as if we are saying "this is what will naturally be happening at that time." When we use the present continuous for the future, we are saying with fair certainty that we are planning to do something ("I'm meeting my friends at seven.").

If you want to see some comments I made about the future continuous tense last year, please click here.