1. I have forgotten what I learned at/during primary school.
We use the present perfect when we want to talk about things which happened in the past but are somehow connected to the present.
In this sentence you explain that you no longer, at the time of speaking, remember what you learned at primary school.
2. I forget what I have learned in my primary school.
You would use the simple present to make general statements.
I forget things all the time. (general statement)
I am not a teacher.
Student or Learner