Question

Are there instances when the use of comma before the word “but” is not needed? What are examples of this?

Answer 1

see below

Explanation:

when using 'but' to mean the same as 'except':

'there's nothing here but oranges'
'there's nothing here except for oranges'

when using 'but' to mean 'only':

'there is but one orange'
'there is only one orange'

when using 'but' to show exception:

'the last but one orange in the line'
where 'last but one' means 'second last', or the orange adjacent to the last in the sequence.