"I exchange my coat for your hat."
Translation:J'échange mon manteau contre ton chapeau.
With échanger ("to exchange") always use contre ("in exchange for" - http://www.wordreference.com/fren/contre several meanings depending on context). Literally literally translating as "I exchange my coat in exchange for your hat".
With changer ("to change") use pour ("for") OR contre
As in English, exchange and change have similar meanings but they definitely don't have the same meaning.
You may note in my comment that the first example was j'échange and the the second example was je change.
J'échange takes contre alone as does the correct answer posted at the top of this page.
Je change takes either contre or pour but in conjunction with the appropriate following verb.
Sitesurf says that échanger pour is incorrect. If Duo is still saying it is correct then Sitesurf disagreed if I understood his comment correctly.
Using change and exchange as if they mean the same thing is very common in English, so Duo is doing the same thing with French.
I take you at your word that the French, in ordinary conversation would never use j'échange and je change as if they mean the same thing and break the grammar.
As Duo, Sitesurf and myself have pointed out the correct usage is J'échange contre. As Sitesurf and myself have pointed out je change pour + verb is also correct grammar and mean much the same thing.
I disagree with your point that what English does is irrelevant. This course all about English usage. The example at the top of this page is to translate from English to French. Those English speakers who treat exchange and change as being indistinguishable in English, in the right context, may well be led to think the same can be done in French. While the French might be close in meaning, échanger and changer are subject to different grammar.
It is worthwhile to point out to English speakers that it easy to make a mistake when translating échanger/changer. So easy that it appears that Duo itself has fallen into the English speaker's trap when giving an alternative correct answer.
It is not just a Duo mistake. It is a mistake common to English speakers. Watch out for it. I am not sure why you would object to drawing attention to it.
I think we may be misunderstanding each other just a bit. I completely agree with you and Sitesurf that "échanger" requires "contre" and that "changer" uses "pour" for a similar meaning.
In English both "change" and "exchange" use "for" when they have the sense of "échanger." That is not an error; it is how English works.
But when Duo offers "échanger pour" as a "correct solution," it IS an error. That is not how French works.
I think that what you are saying is that literal, word-for-word translation is a mistake, and again I agree thoroughly. I don't think we have anything to argue about.
Well, again this damned prepositions! Funny thing: if a translate sentence to my native language, it will be " J'échange mon manteau sur ton chapeau." Definitely, i really don't understand sometimes which preposition should be used in certain sentences. In fact it's very important moment. Choosing the wrong preposition can give the wrong meaning to the whole phrase or transform it into meaningless "abracadabra"...
Well bet your bottom dollar that there is an intimate, infinite very involved grammatical reason for it. Some students here seem to have an amazing grasp of grammar and may hit this thread and provide answers which were copiously forthcoming in the "Contre" thread. Since doing this course I learn that my GCE English (sort of second-rate Baccalaureate) gave me a C+ pass whilst being pretty ignorant of grammar. I really struggle with it here. No bad thing though.Never too late but such a shame. FTR I took my exams in the early 1960s.