(i) The opposite of 'thick' is 'thin'. A thick book has many pages, while a thin book has fewer pages. (ii) The opposite of 'long' is 'short'. A long journey takes more time, while a short journey takes less time. (iii) The opposite of 'soft' is 'hard'. Cotton is soft to touch, while rock is hard. (iv) The opposite of 'slow' is 'fast'. A tortoise moves slowly, while a rabbit moves fast.