Gens vs peuple
Are there any rules/ guiding principles about when to use which? Both appear in the People lesson, but I can't spot anything obvious.
"des gens" is a group of non-specific persons. = "il y a beaucoup de gens dans ce restaurant." "There's much people in this restaurant"
"un peuple" is a group of ethnic similarities, like "le peuple breton", "le peuple français". = you may think of "we, the people of America" : here we can use "Nous, peuple des Etats-Unis d'Amerique".
You can use "peuple" in meaning crowd but in slang sentence. "Il y a du peuple" = "it's much crowded"
"Gens" are generally plural, as you can't make a group with only one person, unless for specific or jokes purposes. "Peuple" is generally singular, unless you're speaking historical or political matters ("les peuples du monde...")
Quite right, but change "can" to "can't" in your last paragraph.
Fun fact about the word gens : this word is both masculine and feminine, this one word is special (like any second language learner's nightmare ;-)) : if an adjective is placed before, it should be feminine, but if it's after, it's masculine. Just another crazy bit about French language. Examples : "Les bonnes gens", but "Les gens sont bons".
See this link, section "note" for a complete explanation.
Edit : you absolutely don't have to remember this rule. Most native French speakers aren't even aware of it (see 22decembre's reaction to convince you). No one will ever tell you you're wrong if you don't respect this rule, unless you're a paid professional translator!
I am a native french speaker... this rule is so specific I even never learn it in school ! You are referring to something duolingo users will never face (or if they face, it means they are living in the country, so better go to professional courses than just the beginner courses available in duolingo).
Of course you don't learn this in school. This is just a crazy bit of French Grammar and would serve only to mix up children trying to learn proper grammar. I took the precaution to write "fun fact" before speaking about it to indicate it's a curiosity fun to know just as general knowledge, but in no way necessary for anyone learning French. The proof is that a majority of natives don't even know about it and they live perfectly fine without knowing! I may mark it bold to put emphasis on the fact that no one has to remember that rule.
"un peuple" is more specific and concrete than "des gens", it is close to "une population" (e.g. "d'une nation"). Think of its latin origin: "populus". ("Its people have a strong tradition.")
"des/les gens" is more vague and can mean simply a group of persons or every person in general. ("Les gens sont fous, les temps sont flous": http://youtu.be/qIvR-i5F69c)
I have to disagree with that. "People" is the correct plural of "person" in most contexts, although "persons" can be correct if the context if more formal or legal. Here's what Oxford has to say: http://www.oxforddictionaries.com/definition/english/person (scroll down to "Usage")
"People" can also be a collective noun, referring to an ethnic group or a nation - "Peoples of the world..." It is legitimate to use it in the singular, i.e., "a people", but is not often seen/heard. I had to really think hard to come up with an example - there is a well-known book by Howard Zinn called "A People's History of the United States"; similarly, a Canadian textbook is called "Canada; A People's History". (Although, just to confuse matters further, the French title of that book is "Canada: une histoire populaire", not "Canada, une histoire d'un peuple.").
Wikipedia refers to the legal convention of referring to the lawyers representing the US government as "The People", but I have to say that I'm pretty sure that in that context, it is a plural noun, not a collective one. Example: In a courtroom here in Canada, you might hear, "The Crown presents its case", while in the States I believe it would be , "The People present their case".
You're right. But English uses "people" as the most common plural of "person", while in French, you can't use peuple as the plural of personne, they are two different concepts. The French peuple only has that collective noun meaning that "people" can have in English.
I tried to simplify it to make it clearer, but I did make a mistake in the process ("persons" is also accepted, correct English).
Actually, there is a singular to gens : gent (which is feminine), but it is very, very rare and literary. The most known use is a fixed idiom : la gent féminine. But it has a different meaning, it means a specific group of people, and is usually followed by a characteristic to clarify which specific group (like women in the above example).