May I use a paper clip, rubberband, or any other item to bookmark pages in a book on Shabbos? May I fold the corner of the page?
If not, is there any permitted way?
Is it considered Hachana?
We are not allowed to make a mark with our nail in a sefer, because this is considered kosaiv, and because we want to make this mark. However using a paper clip or rubber band is not considered writing and permitted. The same would be to fold the page of the sefer. Even though it will make a mark, since it is not the mark that is wanted, rather the fold in the page is what will be reminding you of the correct page.
Regarding hachana, that is also not a problem because it might be needed for today as there is a possibility that you might want to look it up again.
M:B 340-25, Kinyan Torah 2-11, Toldos Shmuel – Kosaiv 68-3, Shmiras Shabbos Khilchoso 28-16, Rivivos Efrayim 1-223(11), Chut Shani pg. 213. Horav Yisroel Belsky zt”l quoted in alachically speakingHalachocally speaking 2-5, Shevet Ha’kehusi 1:130.