Which is/are incorrect?and why (the underlying rule/concept/the basis is requested)?
1) They have been working in this company since last three years.
2)They have been working in this company for last three years.
3)They have been working in this company since the last three years.
4)They have been working in this company for the last three years.
They have been working at/for this company for the last three years. We use "since" with a reference to a specific point in time. For example, "since 2015".
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They have been working at/for this company for the last three years.
We use "since" with a reference to a specific point in time. For example, "since 2015".
This is incorrect spacing:
2)They have been working ...
It should be like this:
2) They have been working ...