Where was that grandparent born?
Portugal — Citizenship by Descent
Two things have to be true of them: that they were Portuguese by origin, and that they never lost it. Where they were born decides both. Portugal's decolonisation in 1975 took the nationality from most people born in its African territories, while those born in Goa, Damão and Diu, Macau or Timor under Portuguese administration kept it.
Mainland Portugal, Madeira or the Azores → Abroad, to Portuguese parents → Angola, Mozambique, Cape Verde, Guinea-Bissau or São Tomé, before independence → Goa, Damão or Diu, Macau, or Timor → I'm not sure where they were born →
Question 3 · start again