verb TO BE and prepositions

Question

Is the sentence ''I was in Thassos for my holiday'' grammatically correct? is the preposition for place correct or shall it be ON Thassos?

Answer 1

If Thassos is an island, "on". If it's a political area, e.g., county, city, duchy, then use "in".

What if Θάσος is both the name of an island and also the name of it's main city? The choice is up to you, depending on context, e.g.,

Yesterday we landed on Θάσος and stayed in Θάσος overnight.

That choice of prepositions makes clear the difference between island and city.