It doesn't tell if you have to write the singular you or the plural you
Both versions are correct, depending on the context. 'Voi scrieți' = You (all) write a message.
Its hard to remember tences
It doesnt explain why the verb is different
"To Write" : A scrie
I write - (Eu) scriu
You (Singular) write - (Tu) scrii
He/She writes - (El)/(Ea) scrie
We write - (Noi) scriem
You (Plural) write - (Voi) scrieți
They write - (Ei)/ (Ele) scriu
Ei = Masculine
Ele = Feminine
Isn't "mesaj" a feminine noun? I mean, why is "un mesaj" and not "o mesaj"??
Mesaj has the neuter gender, so it behaves as a masculine noun in singular but as a feminine noun in plural. Un mesaj, două mesaje.