My own sentences where "that" = "which":
(1a) The forum (that) I posted this question is for English language learners. — (I know it's) incorrect
(1b) The forum (that) I posted this question on is for English language learners. — (I know it's) correct
(1c) The forum which I posted this question is for English language learners. — (I know it's) incorrect
(1d) The forum which I posted this question on is for English language learners. — (I know it's) correct
from wordreference.com:
(2a) It happened the day (that) the car accident took place. — (where it's said that it's) correct
(2b) It happened the day (that) the car accident took place on. — (where it's said that it's) incorrect
from wordreference.com:
(2c) It happened the day which the car accident took place. — (where it's said that it's) incorrect
(2d) It happened the day which the car accident took place on. — (where it's said that it's) correct but less natural than (2a)
That is, the sentences with "which" are subject to the same rules:
(1с) & (2c) are both incorrect,
(1d) & (2d) are both correct.
But why are the sentences with "that" not subject to the same rules?:
(1a) is incorrect whereas (2a) is correct,
(1b) is correct whereas (2b) is incorrect.
It's said on "wordreference.com" that in (2a-d), for some reason, "that" ≠ "which" but "that" = "on which". To me it's a very strange and unclear explanation because I can't see any differences between (1a-d) and (2a-d) in terms of grammar.
Could you explain to me please the rule:
when "that" = "which" and when "that" = "on which"?
Or more generally:
when "that" = "which" and when "that" = "some preposition + which"?
