"Quel" or "ce qui"?
Hey there fellow Duolingo users! I was trying to write "Do you want to know what a good thing to do would be?" but I don't know if I should you use "quel" or "ce qui". I suppose this sentence would be written as "Voulez-vous savoir ce qui serait une bonne chose à faire" or "Voulez-vous savoir quelle serait une bonne chose à faire" but I don't know which one to use. Thank you very much for your help and have a good one!
Use ce qui.
Quel is usually an interrogative pronoun; when placed at the beginning of a sentence it's used to formulate questions which in English usually correspond to "Which... ?" Even if you were going to use it as a relative pronoun, it would be lequel which means something like "which one" - sorry, there's no direct equivalent in English, but a couple examples of its usage might be dans lequel "in/into which" or Lequel dont je te parlais "The one I was talking to you about".
Salut Arcaeca !
As far as I am aware quel is an interrogative adjective (or exclamative adjective) and que is an interrogative pronoun.
However, lequel is an interrogative pronoun: https://www.lawlessfrench.com/grammar/interrogative-pronoun-lequel/
EDIT: Sorry, there appears to be a problem with your linking to the pdf - you'll need to copy and paste the address into a browser.
So here's where it gets confusing... :)
TL;DR: To sum up my argument, all three words have more than one use. Que can be an interrogative or relative pronoun. Quel can be an interrogative or exclamative adjective. Lequel can be an interrogative or relative pronoun.
Now for the super long answer!
First off, we're in agreement that lequel is a pronoun. That much is unarguably true.
And according to Wiktionary, quel apparently IS an adjective, which makes a little more sense now that I think about it.
Quel is usually used, as I said, to form questions which in English begin with "which". I explicitly said "usually correspond" in my above post with the set phrase "Quel âge as-tu" in mind, which you could translate as either "what is your age" or "how old are you", neither of which begin with "which". This hints at a deeper idea - probably expanded upon in those links - that quel is used to form a question around a direct object. It usually happens to correspond to "which" in English, but that's not actually the rule.
The rule, as far as I can tell, is that quel turns a direct object into the topic of a question, qu'est-ce que acts as the direct object itself (more specifically it's the que at the beginning that's the object; the entire phrase literally means "what is it that...?"), and quoi is a stand-in for any other kind of object, indirect or otherwise - basically, if a preposition needs to be involved, use that preposition + quoi. With objects bolded:
Quel est ton sujet de discussion favoris? -- Qu'est-ce que tu aimes discuter? -- De quoi aimes-tu parler?
And then quel can sometimes, as you said, be used as part of an exclamation, like in quel dommage !
Lequel, on the other hand, can be an interrogative pronoun, but from my experience its use as a relative pronoun is more common. When an interrogative pronoun, lequel seems to replace quel + a noun; I could, for example, ask Quelle langue veux-tu apprendre? or Laquelle veux-tu apprendre? - either is fine, it's just the second is less redundant if we've already been talking about choosing a language to learn. In this case, we usually translate it as "which one".
But it can also be used as a relative pronoun, which is how I see it more often. It takes the place of a previously mentioned noun and - either this is the actual rule or this is just all I've ever seen - is used when you have to use a preposition with that noun. Depending on what that noun is exactly, it might be translated as "which" or maybe "whom". We use it for phrases like "in which", "on which", "with whom", etc. - although, with a person, I'd say it's more common and less confusing to use qui instead of lequel.
And finally, as for que, it can be a stand in for a direct object in a question, in the same way that quoi is a stand in for any object that isn't direct. In this case, it means "what" and is the base of qu'est-ce que. So you can say Qu'est-ce que tu penses? but you can also say Que penses-tu?
But, que is also a relative pronoun, used to join clauses, not replace nouns. It does refer back to a noun, though (the antecedent). You use que when that noun acts as an object in the second clause, and qui when it acts as the subject. Basically, with que there will be a subject change, but not with qui since qui is the subject. Compare L'homme que j'ai vu with L'homme qui était vu par moi.
Wow! I hope you speed-type ;D
After scouring the internet for examples of quel as an interrogative pronoun I found nothing SO I deferred to Larousse: http://www.larousse.fr/dictionnaires/francais-anglais/quel/64867
and the final entry confirms that, when one is implied, it is a pronom interrogatif (obviously taking the place of lequel et al).
de tous vos matches, quel fut le plus difficile ?
"of all the matches you've played, which (one) was the most difficult" OU
"which was the most difficult one ?"
However, the main point of my initial post is that quel is usually used as an interrogative adjective not pronoun.