"This is tomato soup, there are tomatoes inside it."
Translation:זה מרק עגבניות, יש בתוכו עגבניות.
Well, I asked a similar question (about lemonade) in the duolingo app - in my Hebrew club - and an Israeli wrote to the Hebrew language academy and asked, then paraphrased their response. It's because most of the produce (combinations) are influenced by Russian (regarding singular vs plural). With a few exceptions (mostly for Aramaic and Arabic).
If you search "Russian influence in modern Hebrew" you'll find some great links.
In my opinion, as a native English speaker, it is not strictly correct to say, "inside it" (as in, "there are tomatoes inside it".) It should be "there are tomatoes inside OF it." This may be up for interpretation because in the common vernacular, it is often said. I am curious to know what others think.
You can see both have been used throughout the past two centuries (although this is a decade behind so you can't see current usage): https://books.google.com/ngrams/graph?content=inside+of+it%2Cinside+it&case_insensitive=on&year_start=1500&year_end=2009&corpus=4&smoothing=2&share=&direct_url=t4%3B%2Cinside%20of%20it%3B%2Cc0%3B%2Cs0%3B%3Binside%20of%20it%3B%2Cc0%3B%3BInside%20of%20it%3B%2Cc0%3B.t4%3B%2Cinside%20it%3B%2Cc0%3B%2Cs0%3B%3Binside%20it%3B%2Cc0%3B%3BInside%20it%3B%2Cc0
Thanks for your response, however, if people are questioning prepositions why would you think they would know terms such as "nominal usage" (which 90% of people would NOT know)?
Secondly, "inside of" and "inside" are synonymous in American English: https://english.stackexchange.com/questions/64504/inside-or-inside-of