I think it will be more fun for you 1. [to study together] compared 2. [with] 3. [studying alone]. 1. Here, I used an infinitive (to study) as a gerund is not suitable here ("more fun for you studying together" sounds unnatural). I added "to study together" as your original sentence sounds odd without two things for comparison. 2. We use "compared to" to compare the similarities or to liken two things. In your case, you are highlighting the differences. For this purpose, we use "compared with". Reference URL: https://writingexplained.org/compare-to-or-compare-with 3. I changed this portion to a gerund phrase (studying alone) as you are comparing one thing (to study together) with another thing (studying alone). I am sure there are other ways to form a sentence to depict your intended meaning. Mine is merely one of them.

It doesn't make sense because something is missing I will try to correct it but I'm not sure if my guess is right. I think it will be more fun than when you study alone.

I think it will be more fun compared to when you study alone

Thank you very much Lorena

It's technically ok but it has to come across easier to understand for the reader. The way you can make it easier to understand is by adding a bit more information to the sentence. So perhaps you can change it to "I think it will be more fun TO SPEND TIME WITH ME compared to just studying alone". It just depends on what you want to get across with the sentence. : )