"This is my sister whose boyfriend is a teacher."
Translation:Das ist meine Schwester, deren Freund Lehrer ist.
The use of the words "boyfriend" and "girlfriend", implies individuals who are in a particular type of relationship. How should you translate the above English sentence if the individuals being referred to are not in such a relationship?
You'd use something like "This is my sister who has a friend who is a teacher" = Das ist meine Schwester, die einen Freund hat, der Lehrer ist.
You would avoid the "whose friend" (which would tend to imply that she has exactly one friend) and use something like "who has a friend" (which does not imply this).
Do you have to use 'das?'
When you introduce something new to a conversation by "pointing" to it with a word such as "this, that, these, those", you use neuter singular in German (dies, das) -- regardless of the gender of the new thing(s) or how many there are.
Could i not say 'Die ist meine Schwester, deren Freund Leher ist?'
You could not.