That's one of the difficult things about the French language. Just like English, there are silent letters. The situations to use them must be learned from listening to conversation. Take "Moi non plus" (me neither) for example. The 's' is silent in this case. So just "PLU" here. French likes to flow cleanly, so if "plus" is before a vowel, the 's' is said out loud because it flows into the following word. Similar to English as to why "an apple" flows and is correct and not "a apple." So, "la plus importante" (the most important), the 's' is said because it's flowing into the vowel of importante. As for 'tous,' in "Laissez-nous tous" (everbody leave us), you'd say the 's' here. In "Plus tard," the 's' is silent (PLU TARRR). Hope that helps.

I learned French about 40 years ago in Quebec & elision was very much in fashion. That's where a terminal "s" is sounded before a following vowel. I have traveled a lot in French-speaking countries over the years, but only for brief trips, so I don't have an expert ear for the vagaries of the language. I have heard that elision is falling out of fashion in France. When I go there I use the language as I know it and i seem to be able to make myself understood. Here is a web site that deals with terminal letters in French: http://french.about.com/library/pronunciation/bl-lettresmuettes.htm . I hope that helps. I look forward to reading the other answers & comments to your question. Richard