Why is 不 sometimes put between the syllables of a 2-syllable word?
There are only two potential particles, and they are 得 and 不
When a potential particle is inserted between a verb and its result complement, it turn that verb from an active verb into a potential verb
Examples:

看見 = See
看得見 = can see
看不見 = can’t see

~

聽到 = hear
聽得到 = can hear
聽不到 = can‘t hear

When an adverb 不 or 沒 is placed before a verb, it funcutions as  ‘not’ and ‘have not’ respectively.
Examples:

看 = to see
不看 = not to see

~

有看 = have seen
沒看 = have not seen