Q. A person was reading a book and wishes to place a bookmark either by scratching with his fingernail or by folding the page, can he do this on Shabbat?
A. One of the forbidden activities on Shabbat is “writing”. One who writes on Shabbat, even if he only wrote two letters, has profaned the Shabbat.
Making a mark by scratching on wooden boards
In the Mishna (Shabbat 103a) the Sages dispute whether a person scratching on wooden planks would be liable for the forbidden activity of “marking” (a sub-component of writing) or whether the Torah only forbade writing. According to Rabbi Yosei, one who scratches on wooden boards would be liable for having performed a forbidden activity on Shabbat. The Tur writes in his halachic compendium (chapter 340) that the Halacha is not in accordance with Rabbi Yosei, and this is also the decision of Rabbi Ovadia of Bertinoro in his commentary on the Mishna, as well as the view of the Mishna Berura and the Shulchan Aruch and most of the early authorities.
However it is Rabbinically prohibited to make a mark on wood by scratching it on Shabbatr. We must therefore discuss a person who makes a light mark on paper with his fingernail: would this be forbidden by the sages, like one who makes a scratch on wood, or is a scratch on paper not considered to be a mark and is therefore permitted on Shabbat?
The Tur writes (ibid) that a person may make a mark with his fingernail on a book and this is also the view of the Shulchan Aruch (340:5). The reason for leniency in this case is because a scratch which will not last, meaning that over time it will not last, is not forbidden on Shabbat, since the sages only decreed that a mark which would remain for a long time is forbidden, such as a mark made on a wooden board. However a scratch in a book does not last for long and therefore does not constitute a forbidden mark on Shabbat. (It should be noted however that one may not scratch one's skin on Shabbat).
Some of the latter authorities, including the Taz write that one may only be lenient regarding books made from animal hide as the mark will not last for long on them, but regarding those made of paper one should be stringent. However from the Shulchan Aruch it appears that there is no distinction, since in his time there were already paper books but he does not mention a stringency regarding them.
Thus, one may make a mark or a line at the side of the page, since this type of mark will not last perpetually.
With permission of halachayomit.com
