"Boil the tea"
Translation:Bouillir le thé
The infinitive verb form is commonly used to give an instruction to an unknown audience, e.g. warnings, instruction manuals, and recipes. It's called an impersonal command. Check it out: http://french.about.com/od/grammar/a/givingorders.htm
There is no familiar form for this exercise. If you're talking about the informal imperative form, it's "Bous le thé", and if it's not accepted, you should use "Report a problem" next time you get the exercise.
It possibly should be as formal form of the imperative. I put "Faites bouiliir. . ." and it's accepted now.
But of course, everyone knows that boiling the tea is the way to ruin it. . .
Généralement il s'agit d'une seule personne qui fait bouillir le thé, pas une foule; donc "bouille" est correcte.
No, as I explained in a previous comment, the correct form for the second person singular informal imperative form is "bous" for the verb "bouillir".
"bouille" is the second person singular informal imperative form for the verb "bouiller", which is an old verb mostly not used anymore, and has a different meaning.
Is there any way to tell when to use the infinitive and when to use 2nd person present as the imperative form?
Without context, both are correct. Infinitive form is used mostly for lists of instructions, like those you can find in textbooks or exercise books. Imperative form is used for orders and commands (teachers to their students for example), or instructions which are intended for someone in particular (someone describing to his friend how to find the way to his house for example).
I just encountered a similar sentence, "Yes, it is necessary to boil the tea," which Duolingo translated to "Oui, il faut faire bouillir le thé." It wouldn't accept "Oui, il faut bouillir le thé," and after looking through the comments in the discussion, I thought I understood why... But now this sentence is accepting simply "Bouillir le thé," and I'm completely thrown... How do you know when to say what? I thought because there's an implication that someone must make the tea boil (as opposed to "le thé bouille" - "the tea is boiling (itself)"), we had to use "faire bouillir." Is this wrong?
Both "bouillir" and "faire bouillir" exist, they mean almost the same thing and both can be used.
If you want to use "faire bouillir" in this exercise, use "Fais bouillir le thé." or "Faites bouillir le thé." (assuming it's an accepted answer, otherwise you'll need to report it).
Strictly speaking the only difference is that when we talk about something boiled in the liquid, or about the container in which we boil, we should use "faire bouillir", if we talk about the liquid itself that boils, we should use "bouillir". But in common French this kind of distinction are often ignored, and you will probably encounter "faire bouillir" a lot more. For tea, both can be used anyway, it depends whether you consider "tea" being the liquid or the leaves, but again, in common French you'll hear: "Je fais bouillir de l'eau." and not "Je bous de l'eau.". It's not that important unless you use French to write novels or articles, etc...
You can have the list of definitions here: