Here's why:

* Conjugate acid-base pairs differ by a single proton (H⁺).

* When a base accepts a proton, it forms its conjugate acid.

* Fluoride ion (F⁻) is a base because it can accept a proton.

* When F⁻ accepts a proton (H⁺), it forms HF (hydrogen fluoride).

Therefore, HF is the conjugate acid of F⁻.