I saw in some books (Rav Belsky, Arizal) that you are not allowed to learn torah Shebictav (Tanach) at night without commentaries. Why is that?
Mishne Berurah (238, Shaar HaTziyun 1) writes that the Midrash (Pirkei d'Rebbi Eliezer 46 and Tanchumah Parsha Ki Sisa 36) writes that Moshe Rabenu was taught the Written Torah during the day and the Oral Torah at night. From this Midrash the Ba'er Hetev (2) (and Arizal [ed]) concludes that one should not learn the written Torah at night.
The Pri Megadim writes that it is permissible to learn Torah Shebeksav at night and the Shaar Hatziyun concludes that even according to the Machmirim it is only a preference and not an issur.
Additionally, the Yesod veShoresh Ha'Avodah (6:2) writes that with a Peirush there is no problem at night in any event.
Regarding Tehillim (which is the Written Torah and according to Arizal should not studies at night), the Kaf Hachaim (237:9) writes the minhag is to recite it after Chatzos, But Aishel Avraham (Butchatch) writes there is no problem at night but then adds perhaps only with a minyan.
See Dirshu edition of Mishne Berurah, ibid) and also see Yabia Omer OC 17
Some add that Friday and Yom Tov evening is not problematic to learn Torah Shebeksav according to Arizal (Yosef Ometz 54). Some include Thursday night (going into Friday) as well (Yesod veshoresh ha'avodah and others. See Piskei teshuvos footnote 32)
In the siddur of Rabbi Shabtai from Rashkov, which presents kavvanot of the Arizal (many books do, it's just the one I have at hand), it says, in the section 'Kriat Shma she-al Ha-Mita':
For the Kabbalists, part of the rhythm of time is that during the first half of the night the forces of judgment are ascendant in the world of עשיה. Also, for the Kabbalists, different kinds of learning--e.g., learning Mishnah, learning Kabbalah, learning Miqra-- stand in different worlds. Since Miqra stands in עשיה, it shouldn't be engaged in at night.