I changed the four original lines of making squares into a loop of four squares, shortening the code.

No it was not difficult to figure out how to rewrite square using a for-loop. The strategy I used was; instead of repeating forward and right multiple times, I told the program to space out the squares and draw 5 of them.

square.py

@@ -1,17 +1,16 @@
 from turtle import *
 
 def square(side_length):
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
-    forward(side_length)
-    right(90)
+    for i in range (4):
+        forward(side_length)
+        right(90)
+
 
 sizes = [20, 40, 60, 80, 100]
 for size in sizes:
-    square(size)
+    square(20)
+    penup()
+    forward(40)
+    pendown()
 input()