Translation:Our father had made pizza yesterday.
Αυτή η πρόταση είναι λάθος, το σωστό θα ήταν "baked" - είχε ψήσει πίτσα
Oops - after posting the previous I realize that ´μας' is ´our´, not ´us´ - apologies.
The word "yesterday" here is χθες, but I have seen another spelling χτες somewhere. So which one is correct or are they just two different forms? And if so, which one is more frequently used?
are they just two different forms?
Correct. There is another variation: εχθές or εχτές. Also, ψες/εψές (local idiom).
And if so, which one is more frequently used?
I don't think either of the χθες/χτες pair is favoured in everyday speech. If we are talking about more formal situations (including written Greek in news outlets for example), I would expect to see the -θ- variations.
There is also οχτώ/οκτώ, εφτά/επτά and others. I think these variations stem from pronunciation and regional accents as the language evolved. In the meantime, as rules were put into place that defined the 'official' language, some forms, either old or new, must have gained popularity while other maintained their local character and are used to the day.