Q. For someone in the conversion process to Judaism, I realize that they cannot keep Shabbat completely, but in what ways can they break Shabbos? Do they have to perform an Av Melacha? Or does violating something d’rabbonon also constitute breaking Shabbos?

If you could shed some light on this issue, that would be very useful! Thanks so much!
　
A. Two reasons are mentioned for prohibiting a Gentile keeping Shabbos. Firstly, the Talmud (Sanhedrin 58b) quotes as a source the verse (Bereishis 8,22): “Day and night they shall not cease (work)”. A second reason is found in the Midrash (Devarim ch. 1,23) that cites the verse; “Between me and Bnai Isroel, (Shabbos) is a sign forever” (Shemos 31,17). See also Talmud Beitzah 16a.

There are Poiskim who maintain that once a Gentile has decided to convert to Judaism, has initiated the process by consulting a Bais Din, and was instructed to begin keeping Mitzvos including Shabbos, (so he will become accustomed to keeping them), he is permitted to observe Shabbos completely. (Toisfois Yshonim, Y’bomos 48b, Responsa Leib Aryeh 33, Ois Leisroel 34, Zahav Mordechai p.55). It is apparent from the Midrash (ibid.) that just deciding to circumcise, will permit him to keep Shabbos. Some Poiskim however, require the Bris to be already done (see Binyan Tzion 1,91).

To avoid desecrating Shabbos for someone in such a situation, a number of suggestions have been proposed; such as wearing a Taalis in a street that has no Eiruv on Shabbos, or to do work immediately after the end of Shabbos, (Since in the Gentile’s day, the night follows the day).

If we are to follow the reason for prohibiting a Gentile to keep Shabbos given by the Talmud (ibid.) that “they should not rest”, both the Rambam (Shabbos 21,1 ) and the Ramban (Beginning of p. Kedoishim) consider a Rabbinic infringement of Shabbos (a shvus), as a breach in the Biblically ordained rest of Shabbos.

Horav Shlomo Miller’s Shlit”a opinion is that it suffices to do a issur d’rabbonon.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.