In the example in which they sweep the floor before we come, why is it nous ne venions instead of nous venions? Why is ne there?

Hi John,

this use of "ne" is called ne-explétif. It doesn't carry any negative meaning. It is there for emphasis. For it to be negative you would need its accompanying "pas" (or any other of its brethren).

There are several lessons offered on kwiziq on this topic. You can search on the library.

Greetings, -- Chris (not a native speaker).