Quelles and lesquelles are not to be used interchangeably. The reason being is that quel/quelle/quels/quelles all have the meaning of what/which while lequel/laquelle/lesquels/lesquelles take it a step further to mean what/which one. The quel group is very vague which is why a noun is required in the phrase to make it complete. The lequel group could stand alone since the noun that is being addressed is already in context with the word (I believe the le- prefix is the reason why, but I'm unsure)

Votre is in reference to a singular object. "votre maison" = "your house" Vos is in reference to multiple objects. "vos maisons" = "your houses".

In the given translation they use "tes" (tu / ta / tes) the informal version.

Advanced students on this board suggest that they have slightly different meaning. Quelles being which and lesquelles being which ones. In conversation you can interchange them but should consider that the latter carries more focus.

Your manner of speech sends signals about yourself. If you typically speak with precision in English because that is just how you speak, then subtle differences that are more a matter of style than definition merit attention. Some people don't understand or care about the difference between which and which ones in English. Other people do understand or care about the difference and it is reflected in their manner of speaking and writing.

Of course, while their meaning is similar to the point of being indistinguishable, their function is not. Quel/s is an adjective. Lequel/s is a pronoun. Different rules of grammar apply to each.

Duo's system is that they take a phrase in either French or English, translate it and then put together sound tracks representing each language. Once all that is done they then start posing questions using these phrases.

They will take the phrase from one language and use it as a test question expecting to get the appropriate translation. They will also use the same phrase in the alternate language to test in the opposite direction.

Additionally, they will use the sound track of each phrase twice, once in each language. As well, they will pose the same phrase in multiple choice questions. In the multiple choice questions, and occasionally in others, no suggested correct answer is given.

Apparently in an effort to reduce the number of comments pages, each method of posing a phrase leads to the same comments page. Thus each student posting comments on a thread will be responding to a specific presentation of a phrase which may very likely be different from someone else on the same thread.

Sometimes it's difficult to answer questions posted on comments pages because the poster seems to be talking about a different example of the phrase.

In this case, the example I had was exactly like the one at the top. When I answered it correctly, exactly like the top of the page, no other answer was suggested. That means I don't know what the suggested correct answer actually suggested. For that matter I don't even know for sure whether your example was posed in English or French. Mine was English to French.

You are correct that quelles is an adjective (which). You are correct that lesquelles is the pronoun form. You have the word without in brackets in your post as if you consider that to be a definition for lesquelles. If so, that is not a usage that I have seen. What advanced French speakers on this board have suggested that it is good to think of lesquelles as meaning which ones to distinguish it from quelles/ which. If that is what you were wondering if you were mistaken about, then I hope this helps.

Many if not most students post here on he assumption that everybody is starting from the exact same example. But it's clear from looking a lots of comments pages that isn't how it works. This long comment is to encourage you as well as others to try to put more information in their posts.

Lesquelles chaussures sont à vous = which shoes are to you.

As for your question about why it has to be "....a toi?" I'm not sure what you are referring to. The correct answer is at the top of this page.

'ta' is used with singular feminine noun, 'ton' singular masculine, and 'tes' for plural

Ton = your singular masculine informal

Ta = your singular feminine informal

Tes = your plural informal

Votre = your singular masculine and feminine formal

Vos = your plural formal.

Quelles (pl) sont (pl) tes (pl informal) chaussures (pl)

Quelles sont tes (ou vos) chaussures? = Which ARE your shoes?

Quel est ton (ou votre) chaussure? = Which IS your shoe. Does that help?

Because the question isn't an either/or proposition. It clearly tells you to mark ALL correct translations. So really then, if you're only answering half the question, you can't expect to get the question correct. Maybe Duolingo should penalise you half a heart for missing half the answer.

Because "ses" means "his" or "her," not "your." "Your" (when the possessed noun is plural) is "tes" or "vos".

No. "Vos" is "your" (either formal or for the plural "you") for plural nouns, and "ton" is "your" for singular nouns. "Tes" is often the same thing as "vos."

Quelles = which (plural, feminine). Sont = are (third person, plural). Vos = your (more below). Chassures = shoes (feminine plural noun).

The words for "your" in French are: votre, vos, ton, ta, tes. There are two factors to consider when selecting the word to use for "your."

The first is who is possessing something, the "you" for the "your". If it either someone you would speak to formally or more than one person, you use the "your" that goes with "vous," which is either vos or votre. If it is a single person you would address informally, you use the "your" that goes with "tu," which is ton, ta, or tes.

The second factor is what is being possessed. If the thing being possessed is singular, and you are using the "vous" version, then you use "votre," regardless of the gender of the object (votre chien, votre voiture). If the object is plural and you are using the "vous" version, then you use "vos" regardless of gender (vos chassures, vos chandails). If you are using the informal "tu" version, it depends on both singular/plural AND gender of the object being possessed. "Tes" is for plural things, regardless of gender (tes chaussures, tes vêtements), "ton" is for a singular, masculine object (ton verre), and "ta" is for a singular, feminine object (ta robe).

Nope. La chaussure is feminine, as is La botte (boot) But the moccasin and the wellington appear to be masculine. Ho Hum.

The example above is in the familiar (informal as you call it) form. If your lesson was to translate from English to French then both familiar (Tu, Tes form), formal and plural (Vous/Vos form) should be accepted. (Be careful with "Tu". It is best understood as Familiar rather than Informal. One can be both familiar and formal using Tu/Te/Tes but can not be both informal and formal at once).

@ slimz7. I'm very likely over simplifying here but "quel (quelle/quuels/quelles)" seem to work with "Which" and "Lequel (lequelle/lequels/lequelles)" seems to work with "Which ONES". (There's allsorts of grammar stuff going on with their appropriate use which I'm only beginning to understand.) But so far, this simple quide has worked for me.