Question

which are covalent and which are ionic?
• carbon tetrachloride, CCl4(l)
• hydrogen fluoride, HF(g)
• potassium iodide, KI(s)
• magnesium oxide, MgO(s) .

I got that they are all ionic but that doesn't seem right

Answer 1

A is covalent. You could have googled "Is CCl4 ionic or covalent" and found your answer.
B is polar covelent; i.e., slightly ionic, but mostly covalent but actually is a dipole which makes it polar.
C and D are ionic compounds. You could have googled all of these OR read on Google about covalent vs ionic bonds. Im surprised your text/notes didn't tell you. Generally those sources are quite good.

Answer 2

Here is a good discussion about ionic vs covalent compounds and how to distinguish how they are formed.
https://sciencenotes.org/ionic-vs-covalent-bonds/