Tenses

any difference?
Tom will be a teacher.
Tom will have been a teacher.

and

Tom had been washing his car when i came his house.
This one I'd like to know that had tom finished washing his car or not yet when i came his house?

Thanks a lot

Tom will be a teacher - this is normal for the future. It's a prediction, but stated as a certainty.

Tom will have been a teacher - this is "set" in the future, referring to something that was in the past (from that point of view in the future) but not past right now. In another three months, Tom will have been a teacher for five full years.

Tom had been washing his car when I came home. Tom was no longer washing the car, but he had been doing it for some amount of time which stopped quite recently before I came home. Perhaps you can still see the suds and water on the driveway. This is NOT a common tense sequence.

Tom had washed his car by the time I came home. - More common

Tom was washing his car when I came home. - He was still washing when I arrived.

I agree that this might not be the most typical example for the usage of these two tenses, however to me the past perfect continuous does something in this sentence that neither the past continuous nor the past perfect simple can do:  It suggests duration and also that, although the car-washing activity had stopped, there was still evidence of it when I came.  In other words, perhaps the car was still dripping,  maybe he was just putting his car-washing gear away, possibly his clothes were a little wet, etc.


Compare:

When I went into his room, it was obvious that my five-year-old son had been eating cookies in bed.

Though 'eating cookies' had stopped/ended, the evidence of it was still there:  crumbs on the sheets, maybe some remnants of chocolate on his mouth, empty bag of cookies on the floor next to the bed, guilty expression on my son's face, etc.

Kooyeen wrote:

Yankee wrote: however to me the past perfect continuous does something in this sentence that neither the past continuous nor the past perfect simple can do Hi Amy, why not? When I went into his room, it was obvious that my five-year-old son had been eating cookies in bed. This sounds closer in time to went to me When I went into his room, it was obvious that my five-year-old son had eaten cookies in bed. This could have been at any time in the past -- It doesn't sound as "newly stopped" to me. When I went into his room, it was obvious that my five-year-old son ate cookies in bed. Ate sounds more like a habitual past here rather than a single past instance of eating. I think all of them can be used in the same context. It's just that the present perfect continuous emphasizes the fact that he was eating for some time (ongoing action) and it probably hadn't been long since he stopped... maybe he was still eating. What do you think?

«When I asked Amanda, she told me that she had been cheating on her boyfriend.
Now, I'd say she was still cheating»

It's not evident from "she had been cheating". Furthermore, she probably was no longer cheating.

EDIT: "had been cheating" may have been derived (via tense-shifting) from either "have been cheating" or "was cheating". Will the meanings differ? I think, yes, they will...

For you comment to be true it should be "she was cheating":

«When I asked Amanda, she told me that she was cheating on her boyfriend»

--
«When I asked Amanda, she told me that she cheated on her boyfriend.»

I think it's almost equal to the "was cheating" version, no difference in how systematically she did it.

EDIT:
I'd explain it through the notion of (non-)stative-ness. Eating is a process. If you say he is eating you mean he's doing it right now. If you say he eats you mean "reqularly" or "from time to time"

However, "He is cheating" doesn't mean the guy is having good time with some pretty girl just at the moment... So I's say the verb "to cheat" is a bit more stative than "to eat".