"When are we opening?"
Firstly, it's ποτέ, not πότε.
πότε = "when?"
ποτέ has a basic meaning of "ever" -- in a negative sentence, it means "never" much as in English (I have not ever seen him = I have never seen him). In a positive sentence it stays "ever", e.g. in a question Τον έχεις ποτέ δει; "Have you ever seen him?" or Αν τον δεις ποτέ, πες του να ... = If you ever see him, tell him to ....
so the way to conjugate ανοιχτώ is to remove τ and change χ το α γ? Is this the way to do it for all (or most verbs) that end in χτ before the suffix in original form?
ανοιχτώ is passive aorist subjunctive: "(that) I am opened". I don't think it can ever stand on its own but only after a particle that takes a subjunctive form as in να ανοιχτώ "that I am opened" or θα ανοιχτώ "I will be opened".
ανοίγω is active indicative present: "I open".
So this is a regular conjugation: ανοίγω "I open" to ανοίγουμε "we open".
It's also regular to go from ανοίγω "I open" to ανοίγομαι "I am opened", and mostly regular to go from there to να ανοιχτώ "that I am opened" -- the γ turns into χτ (in modern Greek) before rather than γθ. (Ancient Greek would have χθ, and you can still see that spelling sometimes in modern Greek.)