on était content et on était contents Quelle est la différence entre?N'hésitez pas à fournir des exemples de phrases.

"On était content" implies that "on" is an undefined subject, and anything undefined in French is always masculine singular, hence why "content" is masculine singular.

"On était contents" implies that "on" is a group of which the speaker is part, it's equivalent to "we" in English. In a group, if at least one of the elements in that group is masculine, then the whole group will be considered masculine.
In this case, the gender of the speaker is undefined so it's defaulted to masculine. The number though is known since it's a group. Therefore, "contents" is masculine plural.