Does slabi also mean weak?
in English "you are" does not necessarily indicate a plural though
You are right there - it can be singular (tu esți) or plural (voi sunteți) but in Romanian the adjectives must agree in number with the subject.
Hi. I think it is also a correct answer to say: "Du nu esți slabi, Du esți puternici". Isn't It?
No: esți is singular, puternici is plural.
What is Du? I haven't met it in Romanian - do you mean Tu?
DL does also accept the singular version but it took me 3 goes to be consistent and not make mistakes like putting 'ti esți slab'!
How do I know that the sentence is plural or singular? English does not make that difference? You might want to add some clues to distinguish the form you want us to use or give the two options.