。
Please remember what you promised.
Literal
Promise-did thing [obj-を] remember-[request-てください].
[Past-form verb]+ことを+思い出す — 'remember (the fact) that one did X,' where こと packages the past action as a noun. 思い出す ('recall, bring to mind') contrasts with 覚える ('memorize, learn by heart'): 思い出す is retrieval, 覚える is storage.