"The place where I work." or "The place where I work in." ? Which is correct ? Should I add "in" ? I have problem with prepostions.
It is, "The place where I work." To include "in" in the sentence would require changing it to, "The place in which I work", which is still correct but less idiomatic.
What may help to decide whether you need the preposition as in the question, is to use a variation of the basic sentence, such as answering a question:
I work in an office.
Where I work is in an office.
The place where I work is an office.
The place where I work is in an office building.
Then the alternative construction including "in" starts looking strange:
Where I work in is in an office.
The place where I work in is an office.
The place where I work in is in an office building.
The latter grouping becomes more difficult to say, and often this can be a clue that it is not correct, or at least not idiomatic.