Is this correct? "Since the last X years, Y is being used as ...". Meaning that Y has been used during the last X years, but also that it has been used for the first time only X years ago.
this seems incorrect: "Over the last X years, Y is being used as ...". "over" + passive present continuous feels weird for me.
And by using: "Over the past X years, Y have been used as ...", I think, would not have the same meaning as with "since".
Please, advice!
I'd alter the statement to
"Since its introduction X years ago, Y has been used..."
Or, slightly restructured (as some people may feel more natural):
"Y has been used ... since its introduction X years ago."
(You could swap out "first use", "invention", "discovery", etc., instead of "introduction"...whatever you prefer.)
Alternatively, you could go with, "Starting X years ago, Y has been used..."
"Since" (in both preposition and conjunction roles) is more commonly paired with a singular event serving as the anchoring point of reference, and not a time period. "Since" already conveys the duration of time, and simply needs the starting/anchoring point (the event/date which happens to have been X years ago). So just provide the actual event/date (its first use, its invention, its discovery, Jan 1, 1983, etc.).
Per Dictionary.com:
Since
preposition
continuously from or counting from:
It has been warm since noon.
between a past time or event and the present:
There have been many changes since the war.
conjunction
in the period following the time when:
He has written once since he left.
continuously from or counting from the time when:
He has been busy since he came.
The usage of "since" has been an issue for many English learners, for the French, for example, since I know how its equivalent term in French is used. It often causes confusion among many other non-native speakers as well.
In the context in which it's been used here, "since" means "starting from", and needs a specific date, time or period.
Examples:
Since August 1
Since noon
Since the beginning of the 21st century
Therefore, one cannot say "since many years" or "since X years". In similar contexts, since is replaced by for.
Examples:
For (the last/past) ten years, I've been a vegetarian.
We haven't seen each other for almost a decade.
For the most part of 2014, the company made huge profits.
As for the usage of "over", we may say:
"We met over the weekend."
"Can we discuss this over lunch?"
It doesn't mean the same thing as "since", as in the above examples.
