。
We threw a birthday party for grandma.
Literal
We [topic-は] grandma [of-の] birthday-party [object-を] opened [explanatory-んだ].
パーティーを開く ('to throw/hold a party') — 開く here means 'to open, to hold [an event],' extended from its literal 'open' meaning. Used for events: パーティー, 会議, 展覧会. おばあちゃん is the warm familial form. んだ frames the statement as sharing/explaining.