Slow is the antonym of quick. ‘Slow’ means less than the expected speed whereas ‘quick’ means doing something at great speed. An antonym is a word that is opposite in meaning to the given word.
A synonym is a word similar in meaning to the given word. The same relation of ‘opposites’ works between the other two words too. Always means ‘forever’ whereas ‘never’ means ‘not in the present, past or future’.
So, the analogy explains the relation between the two words of the two groups very well.
