Primary means first in ranking of importance, or main. You can also use primary to refer to something that comes first on a timeline. Also, the first five years of school are considered primary, meaning the first years.
Two of my brothers are still in primary school. I am in twelfth grade, but they are in first and second grade.
My primary job is to make the coffee. I have lots of responsibilities at the office, but that is what I do first in the morning.
Primary is also used when referring to elections, only in the USA. It means a meeting of the voters or the guided process of selecting a candidate to run in an election.
The primary is on Tuesday. I hope that we are able to select someone who can beat the other candidate.
This primary will be held at the local school gymnasium. I think it is the largest room in the town and will be able to hold everyone.
The word leading means to be in the lead, or first. You use the word leading to refer to the most important part of something.
The leading video developer is releasing a new game. Every video game they have developed has been a hit.
Leading can also mean to lead, or guide. In this case, it refers to someone or something in the act of leading.
Susan is leading the way. No one else in our group knows where they are going, and I do not want to get lost.
Our school is leading the nation in test scores. Because of great teachers, our students were able to master the material.