you are taking a big risk driving so fast 中taking 是动名词还是现在分词?driving换成to drive 可以吗有说可以，有说不可以的，希望能说详细点，谢谢！

可以

是动名词.
driving换成to drive 可以.

taking是are taking: 现在进行时，不能换;
如果换，那就是将来时了,
You are to take a big risk...你就要冒险……
driving，可以换：take a risk to do sth是正确结构。
或应该说：take a big risk of doing sth更合理。
Whoever you are, don't take the risk of doing a job like that.
无论你是谁，都不要冒险做那样一份工作。 [朗文当代]。
Those who are not so pushy , they keep in
the background, even though they feel power of a thought, they do not take a
risk to say it.
那些没有进取新的人，他们呆在幕后，即使感觉思维的能，他们不冒险表达它。

taking 为现在分词，构成现在进行时，不可以换成to drive；you are taking a big risk driving so fast 你开那么快是在冒险。

taking是现在分词和前面的are构成现在进行时态.后面的driving不可换成to drive这时一种习惯说法.即take a risk doing sth.=risk doing sth也就是说它们是固定句型.