Learning in a Sukkah:[1]

One must learn Torah inside the Sukkah.

Learning in a Beis Midrash: (However if he is learning inside the Beis Midrash he is not required to enter the Sukkah).

Learning outside under fresh air: Likewise if by learning outside one is able to delve into the subject and understand it in greater depth then he may learn outside the Sukkah in order so he have a more serene mind, as the air of the outside is good for a person to refresh his mind. Nevertheless it all depends on the situation, as if he is able to learn comfortably in his Sukkah then he must do so.

Difficulty moving Sefarim into Sukkah: If he requires many Sefarim for his learning, then if he is able to make space in his Sukkah in a way that he is not required to remove the Sefarim from the Sukkah during the meals and upon sleeping, then he must set up the Sefarim in his Sukkah and learn in the Sukkah. However if he is unable to easily prepare an area for the Sefarim and it takes a lot of trouble to remove the Sefarim during the meal times and then return them afterwards then he may learn outside the Sukkah.[2] 

 

Summary:

One may learn outside his Sukkah if either:

1. He desires to learn in the Beis Midrash
2. He desires to learn outside under fresh air for greater understanding
3. He does not have room to store his Sefarim in the Sukkah

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[1] 639/4

[2] The reason: As all matters of exceeding difficulty of which a person would not bother to trouble himself within his own home he is not obligated to do so in his Sukkah. [ibid]