Strictly grammatically, την becomes τη when it is not followed by κ,π,τ,ψ,ξ,γκ,μπ,ντ,τζ,τς or a vowel. So it's τη σοκολάτα. But colloquially, you can use την with every letter that follows, but not τη. So (colloquially) you can say την σοκολάτα, but never τη πέτρα. But strictly grammatically the rule above must apply.
The verb to eat is τρώω, there is no other verb with that meaning. It is very irregular and takes forms that are not easily associated with it if you don't speak the language, like έφαγα, but it's the only verb that means to eat. Other associated meanings, like consume, wolf down, gobble have their own equivalents. So, if you get a chance to find out, please let us know what that 'other verb' the person you met mentioned, because this does not really make any sense.
Is the morphological rule about the ν from Triandaphyllidis's Grammar? (την becomes τη when it is not followed by κ,π,τ,ψ,ξ,γκ,μπ,ντ,τζ,τς or a vowel.) My understanding is that the ν is present in more formal Gk no matter what letter follows. I'll try to memorize the rule. Thanks for the mnemonic phrase.