Of course!
'Certo' — CHER-to. Stress on the first syllable. 'Che sì' — ke-SI. The 'che' is unstressed.
Use to confirm agreement with obvious or rhetorical questions. Often implies slight surprise that the question was even needed — 'of course, isn't it obvious?'
'Certo che sì' (certainly yes) is stronger than just 'certo'. Adding 'che sì' makes the confirmation more emphatic and sometimes adds a hint of good-humoured exasperation — as if the question shouldn't have needed asking.
Ma certo!
But of course!
'Ma certo' — adds 'ma' (but) to increase emphasis. Standard Italian enthusiastic affirmation.
Ma figurati!
Of course! (Don't even mention it!)
'Figurati' — agrees enthusiastically while dismissing the need to ask. Very warm.
Come no?!
Why not?! (Of course!)
'Come no' — literally 'how not?' — Italian rhetorical structure for emphatic affirmation
Italian generosity in granting favours — signalled by 'certo che sì' and 'figurati' — is a social performance as well as a genuine offer. Even when someone might be slightly inconvenienced, the Italian code of friendliness requires an enthusiastic 'certo!'