"You must eat more."
Translation:Vous devez manger davantage.
'Il te faut manger davantage' would also be acceptable but it is not
What is the difference between advantage and plus? Do you use them in different situations?
No difference in this context. You may want to use "davantage" instead of "plus" if there is any risk of confusion with "ne... plus" (although pronunciation of the later is plu, while the former is pluss), particularly in speech, because we very often skip "ne".
ex: "j'en ai plus" : only the final SS sound should mean that your mean "more" and not "any more" (pronounced PLU)
"plus" when it means "more" is pronounced with a final S sound.
whereas "ne...plus" when it means "no more" is pronounced PLU
What's wrong with Tu dois plus manger? I thought adverbs went after the modal auxilliary.
"More" modifies "manger" rather than "dois". What you propose is not incorrect, but less natural.
Because with that you are expressing a generality, not a statement about your interlocutor. For making it correct you need to add a "te" between the "il" and the "faut", Which translates to "It is necessary TO YOU"
"encore" would be possible, even if there might be an ambiguity with its second meaning: again.
i put "tu dois davantage manger" and got it wrong. is it just word order that lost me a heart or is there something else wrong i'm not seeing?
Generally, adverbs are placed after the word they modify, ie "manger davantage" (= eat more)
I'm probably wrong but I thought I read somewhere that adverbs containing three or more syllables could be placed " at the beginning , in the middle or at the end of a sentence ." I also translated this sentence as " Tu dois plus de manger " Marked wrong though .
Dois is the conjugated verb (linguistically known as a modal verb), so a verb after it must be in the infinitive, very similar to pouvoir and vouloir. Per ex) Je dois y aller maintenant. (I have to go there now.); Voulez-vous parler français? (Do you want to speak French?); etc.