How should i know that 'Zitrone' is singular?
Because you haven't learned the plural form yet. It's Zitronen.
The hover hint says lemon not lemons.
This is bad English, though. That is what led me to think it was a plural form of lemon, although yes, I hadn't learned the plural form. But in English we'd definitely either add an indefinite article or type 'lemons.'
These plurals are killing me...
Hmm, what indicates that this is plural?
The 'e'. Singular: 'Ein Rezept mit Zitrone', Plural: 'Rezepte mit Zitrone'.
Remedies with lemon is wrong?
"Remedy" normally does not translate to 'Rezept', it only does so in very specific contexts.