Chemistry 1 question on acids/bases

[MelonBar]

How can I work out if something is a Lewis acid or base? According to the prescribed text (McMurry) a Lewis acid is a molecule that can accept an electron pair. In other words that molecule has a vacant orbital, which can accommodate the electrons received from an electron pair donor (Lewis base).

For TiCl₄, I marked that as Lewis acid, which is correct. However I'm not sure if my working out is right at all. I got the electron configuration for a titanium atom (1s²2s²2p⁶3s²3p⁶4s²3d²), and reasoned that the 4 covalent bonds then, each containing a pair of shared electrons, would give an electron profile of [Ar]4s²3d⁶ for the bonded titanium. This leaves 4 half-filled 3d orbitals, which can then be used to accommodate an electron pair. Ok... makes sense.

But this logic falls apart for HF, another Lewis acid according to McMurry. The electron configuration for a standalone F atom is 1s²2s²2p⁵. The bond to H then means that the electron configuration for this F in HF is 1s²2s²2p⁶, in which there are no free orbitals except the higher level ones 3s and above.

I thought then all this must have something to do with the valence bond theory (hybridization etc) but it doesn't work for all Lewis acids.

Could somebody explain this "basic" org chem concept to me?

Ty