Foreigners often make some strange mistakes in Chinese when they study the most difficult language--- ---Chinese. We try to analyze typical sentences to correct these errors.

         Sāntiān nèi, nǐmen ké yǐ bù kéyǐ zào hǎo jiàn?
False: 三天内，你们可以不可以造好箭？
         Sāntiān nèi, nǐmen nénɡ bu nénɡ zào hǎo jiàn?
True: 三天内，你们能不能造好箭？

“能”和“可以”混淆。“可以”只强调有能力，“能”既强调有能力，还要有条件。句中询问三天内是否造得好箭，要求不尽造箭的人有能力（技术），还应有某种条件（如原料、工具等）。此句应该用“能”。
Here, 能 is confused with 可以. 可以 merely stresses capability, but 能 emphasizes the objective conditions as well. In the present sentence, asking whether the arrows can be finished within 3 days, to achieve the goal depends not only on the capability （the skill） of the arrow maker, but also on such conditions as material and tools. Thus 能 is the proper word to be chosen.