Genitive case. Particularly, genitive possessive. Elem Heb. I in college....day 2 of class! בית becomes בת as in בת כסא "house of the chair". LOL A euphemism for the toilet. Also works for more straightforward relationships of possession: בת ספרים библютека House of books, lit., Library, I would presume. Let me know if that isn't the case. I mean, if that is "bookstore" or something else.
If I got it right, it doesn't work like this, in German it is a new compound word, the last component of which (supposed the main main to define the type of object under description) defines the gender of the whole compound, whereas in Hebrew it is a smikhut construction, somewhat like English "of"-construction, where each of two nouns retain their inherent genders AND number, here (a) soup of tomatoes . BTW in Hebrew it is the first word, that might change its grammar form while in smikhut, I know for sure, that masculine plural looses -im ending
I don't have this very well understood myself, but I know that we forgo shel because the nouns are put into a construct state. It basically means "soup (of) tomatoes". What I'd like to know is if the noun 'tomato' was put into plural because of the phrasing I just posted, or if it changed to conform to the rules of smikhut (construct state).