In what situations am I actually liable to pay HDB levy - BTO to BTO and Resale (grant taken) to Resale / BTO only?

You need to pay the resale levy whenever you sell a subsidised flat (i.e. with grants), and buy a second subsidised flat or a new EC.

You don't pay it if you're buying an HDB resale flat, or a private property. ​​​

You need to pay resale levy only when the HDB you are selling is a BTO or new EC after 2013 when you bought it or you bought a resale HDB using housing grants, and then buy another same kind of houses (BTO, new EC, or resale with housing grant)