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.
Troll's post from four years ago (2nd from top of page) spells out the 'official' rule, namely that "τη σοκολάτα" here is correct and "την σοκολάτα" is not (officially) correct.
AND YET, "τη σοκολάτα" is still, 4 years later, being marked as wrong here.
Γιατί, φίλοι μου, γιατί;
Let’s accept both. And let’s stop penalising learners for using the (more) correct version. Same goes for « τη λέξη » and many other instances here on DL which are so cruelly rejected.
Question for Greek native speakers : I know people (sometimes) WRITE things like «την σοκολατα » but do you often hear people SAYING the ‘ni’ here ? (To which you are entitled to reply « We are the Greeks who say « ni »! »).
For anyone who wants to get this sorted in their minds, here is a useful link to a Wiki page in Greek on the topic (you can run the first few lines through GoogleTranslate to get the gist): https://el.wikipedia.org/wiki/Τελικό_νι
And here is a much more readable version for kids, saying the same thing: http://ebooks.edu.gr/ebooks/v/html/8547/2009/Grammatiki_E-ST-Dimotikou_html-apli/index_B4e.html