Consider the following distributional facts:
[1] "too" and "very" modify the adjective "cold", and "not" negates the verb:
It
is not very cold outside.
It
is not too cold.
[2] "that not" works in questions, where the subject ("that") and the verb ("Is") are inverted. "not" still modifies the verb:
Is that
not what he said? <question>
That
is not what he said. <statement>
[3] "this not" doesn't work. There isn't a verb for "not" to negate.
[4] "so not" works if there's a comma after "so":
So,
not that he would
notice. <"not" is separated from the verb "notice">
So that he would
not notice. <"not" modifies the verb "notice">
Hope that helps.
