Page 479 - Makino,Tsutusi.DictionaryOfIntermediateJGrammar

soko del / soko de2 405
Second, sore de is used to mean 'that's why.' Soko de cannot replace sore
de in this use, as in [3].
0 * -€
[31 a. @Elbk~%B$133 Lk, {?hT/*?LTl %@B&wEC~T
to
(I had a cold yesterday. That's why I took a day off from
school.)
HHB* LI '
b. A: @Elbk%~-3kk~Tffl&d'djcl$LTb,
(I had some business in Osaka yesterday.)
B: &&, {?hT/*?LTl bab7 L.t.bG~71:/vT.fbo
(Oh, that's why you were not here.)

Third, when sore de is used, the situation does not have to be a special one,
as in [4]. (See Note 2.)

Fourth, when sore de is used, the situation does not have to be controllable,
as in [5]. (See Note 3.)

soko de2 =f LT conj.

then; at that time
event and an action taken at the
time of the event