ᐅ The Tärnan is difficult to operate.

I bought and installed a Tärnan mixer tap. Unfortunately, it is so stiff to turn that the screws under the sink keep loosening. Is this a defect, or did I possibly do something wrong?

I don’t see what could go wrong. Remove the fitting and check if the spout can be easily swiveled when it is detached. If not, then exchange it.