Neutral pronouns le, l'

Question

Here are two examples from the lesson:-Est-ce que tu sais marcher sur les mains ? - Non, je ne sais pas le faire.Do you know how to walk on your hands ? - No, I don't [know how to do that].- Voulez-vous prendre pour épouse Marie Lise Anaïs Lambert ? - Oui, je le veux.- Do you want to marry Marie Lise Anaïs Lambert? - Yes, I do.
Based on the last example ("Oui, je le veux"), why can't you, in the first example, simply use the same construction, and say "Non, je ne le sais pas"?
In other words, when do you add in "le faire"? Is there a rule??

OK. I see that this has been asked and answered below. I still think that you could use either construction: (a) Non, je ne le sais pas. (b) Non, je ne sais pas le faire.

Similarly, it looks as if you can use both forms with the first example too: (a) Non, je ne sais pas le faire. (b) Non, je ne le sais pas.

Accepted Answer

Bonjour Owen,
In the context of taking your vows, in English you would use the phrase "I do" while in French you'll use "je le veux". This is just the phrase you use in that context (using the neutral verb faire would sound extremely strange!)
Also, as Marteen explain, French tends to be a bit more precise than English in this sort of context. 
When saying "No, I don't [know how to do that]", you would use "je ne sais le faire" if the idea was introduced in the question using an infinitive (at least in most cases):
Tu sais marcher sur les mains ? - Non, je ne sais pas le faire. = Do you know how to walk on your hands? - No, I don't [know how to do that]. 
Les dauphins sont des mammifères. - Ah bon ? Je ne le savais pas. = Dolphins are mammals. - Oh really ? I didn't know that.
I hope this is helpful.
Bonne journée !

Answer

I don't see this stating that you cannot make the statement without 'le faire'.

In everyday and informal conversation, and when there is no doubt as to what you mean, 'le faire' may well be dropped, and is quite commonly in my experience. Indeed, as in English the response may be decidedly shorter !

In more formal speech, and in cases where it may be unclear as to what it is you do or do not know/want etc, adding 'le faire' clarifies that it is the 'doing' you don't know, not something more general about the context. You could be asked about doing 'something' that you don't know how to do, and not want to leave the impression that you have actually never even heard of 'doing it'.

I think there is a risk of overcomplicating this - it is not really that different in English. In English we can and do use either construction - 'no, I don't know how/don't want to' or 'no, I don't know how/want to do it/that'.

As French generally leans more to avoiding ambiguity than English (in my view), adding clarity may be more common, but is by no means universal in everyday settings.

As the lesson states : "Note that in many such cases, you add the neutral verb faire to refer to an action".