Hopefully the following isn't too confusing.
The big difference between "saan" and "nasaan" is the little part "na", haha. The particle "na" (and its negative "wala") expresses the idea of 'to be (not to be) in a place'*. This "na" is part of other words like "narito" ("nandito"), "nariyan" ("nandiyan") and "naroon" ("nandoon") which indicates where something is, was or will be. The difference between "saan" and "nasaan" is similar to the difference between "sa" and "nasa".
*from "A Grammar of the Tagalog Language"
From the TDC lesson,
www.tagalog.com/less ons/lesson.php?lesso n_id=62&csid=139. Examples are provided in the lessons.
The question word “where” can be translated into two different Filipino words. Depending on the context, it can be translated into saán or násaan.
Násaán
The question word “násaan” is used when you ask for the location of a person or thing (place/object/animal).
Saán
On the other hand, when you ask for the location of an action, you use the question word “saán”.
Mainly taken from the ETG,
learningtagalog.com/ grammar/detailed_con tents.html, examples are provided on the relevant pages of ETG:
"Sa" indicates location or direction (“in, on, at, into, onto, to, towards, from, through etc.”) or possession ("with").
"Nasa" indicates where someone or something (place/object/animal) is. It is equivalent to (is/are/was/were) in/on/at/with/etc". (Note that "na kay" is two separate words but is like "nasa" that has been combined into one word from "na sa", as I understand it)
Both "nasa" and "sa" may be used to indicate where a place is. Sa may also be used to indicate where something happens.
"Saan" - in, on, at, etc. where, which (place/s), what
"Nasaan" - where (is/are/was/were), which (place/s) (is/are/was/were)
Both "nasaan" and "saan" can be used to ask where a place is.
Further references:
languagecrush.com/bo ok/1
Anyone, please correct or clarify.