Why should I say 'to lease a plant for nine years' but not 'to lease a plant on nine years' for example?

Answer 1

it's FOR nine years because proper English says it's not ON nine years.

when you use ON, it gives the meaning that something is stacked onto something else. FOR gives the meaning that it is done for a certain period of time or for somebody.

hope you can get a clearer picture :)

Answer 2

I assume the plant you're talking about leasing is a factory, not a photosynthesizing green thingy of interest to botanists and horticulturalists.
Anything done (in this case, leasing a property) during a period of time (in this case, 9 years) requires "for" as the preposition before the time description..
Doing something "on" a period of time simply doesn't make sense.
A contract is being maintained FOR a length of time.
Here's why you might confuse the two options:
You CAN say somebody has a 9 year lease ON a property. If you say it passively like this, with the time duration mentioned first, then you can get away with "on" as the preposition (notice that "lease" is no longer a verb but a noun if you word it this way).
If you mention the time duration at the END of the sentence as in your original example...then it has to be "FOR nine years" (and "lease" is used as a verb).