Ok, so what I understand is that Que refers to the direct object, Quoi refers to the indirect object. So when you are to ask "What?" generally you are referring to the indirect object because... you don't know what they said!
But overall don't say "Quoi" unless you are familiar with the person. "Comment?" is much more polite, or "Comment vous dites" is most polite.
"que" cannot stand alone nor at the end of a question.
"quoi" is used as a stand-alone or at the end of informal questions.
- que dis-tu ? (formal) = tu dis quoi ? (informal) = qu'est-ce que tu dis ? (standard)
The more polite "comment ?" is used to ask someone to repeat what they just said, and it is short for "comment dis-tu ?" which is idiomatic and less formal than "que dis-tu ?".
In English, "what?" should naturally and directly translate to "quoi ?" as a general question prompting an answer in the form of a piece of information.
The following dialogue is possible, where "comment" would not be correct:
- A: Sais-tu ce que j'aimerais ? = Do you know what I would like?
- B: Quoi ? = What?
- A: Un gros gâteau au chocolat =A big chocolate cake.
"Comment" is a polite way of saying "What?" Pardon is self-explanatory. "Quoi" is familiar "What?" Think about it in english. We usually say "I'm sorry" instead of "What?" as it can be quite rude.
"Que" is not found by itself at all in french (correct me if I'm wrong). Although it generally means "that" when in the middle of the sentence, when at the beginning of a sentence it means what + a verb (usually with inversion). Ex. "Qu'est-ce que....", "Que manges-tu?" However, it can me "what" in the middle of the sentence if used with "ce." Ex. "Je veux ce que tu as."
Since "quoi" is an object, if a verb ends in a preposition, you can start a sentence with the preposition + quoi. Ex. "À quoi ça sert." Think about "pourquoi" here... "for what" has a suspiciously similar meaning as "why."
It is interesting. Reading the french way of putting words together makes me realize how much English has been altered to simplified speech. For example, using ce + que would be originally translated as that which "Je veux ce que tu as = I want that which you have" rather than generalizing to 'what'. Just a side note. :-) I am enjoying this App much more than I had expected to.
I ain’t not no expert or nothin’, but let’s try and clear the air a bit… In English, we use to word “what” to mean a lot of things: 1. What is that? (direct object). 2. What are you talking about? (indirect object). 3. What time is it? What book do you want? (which), 4. What a good book! (exclamative adjective) or just 5. What? (pardon?) … Among others…
French has different words for these:
- “Que”, as seen in “Qu’est’-ce que c’est?” (What is that?) for example, is ‘WHAT’ being used as the DIRECT subject. “Que dit-il?” (What does he say?)
- Quoi is used for an INDIRECT object, for example “A quoi pensent-elles?” (What are they thinking ABOUT) or “Avec quoi a-t-il coupe le pain?” (WITH what did he cut the bread?). These "with"s and "at"s and "about"s etc. are called prepositions.
Notice these were not subject/object distinctions, but rather differences in how directly they were referred to.
[formatting isssues with Duolino, please refer to the [numbers] from here on]:
 Quel? (as well as quelle, quels, and quelles, depending on the gender/number of nouns) is like saying which: “Quel film tu recommanderais?” = (Which film would you recommend?) ... "Quels sont vos favoris?" (Which are your favourites?)
 BUT also can be used with a preposition “A quelle heure part Pierre” = “[at] what time does Pierre leave?” ...
 Exclamative adjective… for example, “Quel livre interessant!” (What an interesting book!) Or, “Quel rat!” (what a rat!)
 What? As in Pardon? Is “Comment?”
Any questions? On second thought, I don’t think it’s any clearer …
Sources: Hawkins Towell. French Grammar and Usage. 2001, 2nd ed. and Lawless, L. French.about.com/od/mistakes/a/what.htm. 2014.
It should be according to this paragraph: And finally, when you didn't hear or didn't understand what someone just said and you'd like them to repeat it, use the interrogative adverb comment , which is considered nicer than saying quoi (the only reason I've ever heard for this is the latter sounds like a duck quacking.) http://french.about.com/od/mistakes/a/what.htm
Quel is an interrogative French adjective that means which or what and has four forms: masculine singular (quel) and plural (quels), feminine singular (quelle) and plural (quelles). For example, quel jour sommes-nous? (What day is it?) Quelle heure est-il? (What time is it?) Quels fruits aimez-vous? (Which fruits do you like?) Quelles sont tes couleurs préférées? (What are your favorite colors?)
The French words « que », « qui », « quoi », « quel(le) », « quand », « où », « comment » don't always match "that", "who", "what", "which", "when", "where", "how" as you would expect.
« quoi » means "what", but you can't use it everywhere where you'd use "what" in English.
Here is an article explaining the many ways to translate "what" by French language expert Laura Lawless: https://www.thoughtco.com/what-in-french-1369498
There are several contexts where "quoi" or "de quoi" can be used.
As a way to have someone repeat what they just said, none is polite.
But both can be grammatically correct.
- Devine ce que j'ai fait hier soir (Guess what I did last night)
- Quoi ? (= What... did you do last night?)
- Tu sais, il m'en a parlé... (You know, he told me about it)
- De quoi ? (= About what... did he tell you?)
I just got this as a "select all correct translations" exercise, and had to choose between "Quel?", "Que?", and "Quoi?". I have to be honest and say that the only reason I correctly guessed "Quoi?" was because I thought of the French knight scene from Monty Python and the Holy Grail.
"Fetchez la vache."
"Fetchez la vache!"
Obviously not the best source of knowledge about the language, but, hey, it worked this time.