"The good fact"
Translation:το καλό γεγονός
I see you know French already.
It is like with le and la in French -- all nouns in Greek have a gender, but Greek has three genders (masculine, feminine, neuter). So the article can be one of ο, η, το.
You may wish to learn new nouns together with their article, e.g. το γεγονός rather than just γεγονός.
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No -- both are singular.
καλό is neuter singular and καλός, masculine singular.