1. I will do the work by 5 o’clock.
It means that we will be able to say "I'm doing the work." at some moment before 5 o'clock inclusive. That is we don't mean the work will already be done/finished by that time.
2. I will have done the work by 5 o’clock.
It means that we will be able to say "I have done the work." at some moment before 5 o'clock inclusive. That is we mean the work will already be done/finished by that time.
Thus I look at 1. and 2. as at the absolutely different sentences. But as far as I'm concerned, for the native speakers they are identical and both mean the work will already be done/finished by 5 o'clock. Hence my considerations are wrong.
So, could you explain to me please why my logic is not correct?
Thanks!
Best Answer
To "do the work" means to complete the work.
Macmillan Dictionary
If I say, "I will do my homework before dinner," I do not mean I will begin working on my homework before dinner, I mean I will start and finish my homework before dinner.
In other words, we do mean that the work will already be finished by that time.
The two example sentences mean almost the same thing. The promised outcome is the same - the work will be finished. The first emphasizes the doing of the work. The second emphasizes the (completed) state of the work.