In my Oxford Latin Course 2 chapter 21 there is a sentence 'paucis post diebus ubi Orbilius pueros dimisit'. Why is it not 'paucos post dies'? I remember that the preposition 'post' + accusative, doesnt it?
In this case, post is being used as an adverb, not a preposition; essentially, it's being treated as a comparative adverb*, and the ablative paucis diebus is expressing the 'degree of difference,' as it can do also with other comparative adverbs and adjectives**. So literally, the phrase means 'after by a few days.'
That said, paucos post dies, where post is a preposition patterning with an accusative, also works and has equivalent meaning.
* Just as the English word 'after' is, in origin, a comparative.
** For example, multo felicior, 'much happier' (literally, 'happier by much').
Confusion between accusative/ ablative can occur with other expressions of time. The duration of a period of time, i.e. "how long" it lasted, requires the accusative. Occasionally the preposition, per, is used:
for two years = duos annos;
for many days = (per) multos dies.
But for "how long ago" something happened, you use the adverb, abhinc, normally with the accusative; but, it can also be used with the ablative, though this is less common:
two years ago = abhinc duos annos;
three days ago = tribus abhinc diebus.
Perhaps you are already aware that the adverb comes after at least one of the relevant words, in the ablative?