in general， we say 虽然上个星期我没有锻炼，可是我努力吃健康的食品了。And 做完了我的第一个Wii《My Fitness Coach》游戏的锻炼以后，我几乎走不动， 甚至下楼时很痛。

your understanding of those two words are correct, but it might not be suitable for that sentence. Instead of saying 可是当然, you might want to say 不过 or 但是 to express the idea that even though you did not do any execrises, you still try to keep health by eating healty food. saying 虽然上个星期我不能锻炼，不过/但是我努力吃健康饮食了。 虽然....但是 are commonly used together rather than 虽然....可是当然, the later one is rarely used. I baidued the 可是当然 and find out that most of those uses of this term are not very completed and quite easy to make people confused. If you want to say but of course, you might go for 但当然. However, I don't think it is commonly used in oral and writting by natives since the "当然 has already been implied in the usage of 虽然...但是” . For the second one, this phrase is surely used quite common, but its attribute and meaning is more closely to verb rather than noun. 走下楼梯 is used to refering an action, such as an instruction telling you to walk down the stair. 下楼梯 or 下楼 is a noun, so you might want to use "下楼梯/下楼“ in this situation since you are talking about a situation. saying 甚至(连)下楼梯都很痛. Using 甚至(连)……都 at here can emphasize that even such a easy thing like walking down stair will make you feel painful. Besides, 甚至连...都 is also commonly used together. But if you are using them in oral, surely no problem with using 走下楼梯 or 下楼梯.

就像英语当中but和though不能连用一样，不过，书写时，你可以在可是和当然之间加逗号，口语的话，就做出明显的停顿就好了，希望这些对你有帮助

So why is this girl saying I can't say it, and if I can't say it, how do you say ''to walk down the stairs' '(walking downstairs)'.in Chinese? “走下楼”: 是强调正在走的这个运作,她一边走下楼梯一边向他致敬。 而“下楼”在这儿你是要说明当时的感受，是在“下楼时”的感受。 只是着重强调的部分不一样罢了。 "虽然...但是" 是中文的固定用法.

Hi, there! A1. "可是" and "当然" are two high-frequency vocabulary in Chinese, but we don't put them together when we talk, like "可是当然". "可是" means "but" as you know, and "当然" usually means "of course" OR "but of course". "but of course" here is more closer to "不过". It means something I ***(not)***, but I *** other thing instead. e.g. 虽然上个星期我不能锻炼，可是当然我努力吃健康饮食了。 虽然上个星期我不能锻炼，不过我努力的健康饮食了。 A2.You want to express the movement of "getting down the stairs". So, "走下楼梯" is OK here, but too formal. We usually use "下楼梯" for short when we talk. e.g.我几乎走不动，甚至走下楼梯很痛。 我几乎走不动，甚至下楼梯都很疼