Read these sentences, the words in brackets make the meaning fuller.
a)I wish I knew how to drive a car. (At the time of speaking, the speaker doesn't know how to drive a car.)
Or: I don't know how to drive a car. I wish I did.

b)I wish today was a holiday. (But today is not a holiday.)
c)I wish I had more time to read. (At the time of speaking the speaker hasn't got much time to read.)
d)I wish I could travel round the world. (At the time of speaking, the speaker can't travel round the world. Perhaps he hasn't got enough money, or time.)
e)The father told his children, "I wish you wouldn't make so much noise. I've got a headache." (when the father says this, his children are making a lot of noise.)