This is the 827th most frequent Japanese word.
事前
"Prior" or "in advance."
In this sentence, "事前に" is used to mean "in advance," highlighting the necessity of prior preparation in the activity.
事前に計画を立てるのは重要です。
It is important to plan in advance.
In this example, "事前に" once again acts adverbially, emphasizing the timing of obtaining permission as being prior to another action.
事前に許可をもらうことが必要です。
It is necessary to obtain permission beforehand.
Here, "事前の" functions as an adjective to describe the contact, indicating it should occur before an event or situation.