Some Question About Left Join On Condition?

Left join on condition has no effect in the left table itself?
I have two tables:
one named student

studentID studentName
-------------------------------------------------- --------------------------------------------------
1 Jom
2 Jim
3 Tom
4 Kate
5 LinDa
6 DaiAnna


The other is grade table

studentID subject grade
-------------------------------------------------- -------------------------------------------------- -----------
1 math 90
2 Chinese 90
1 Chinese 80
3 math 98
4 math 76


And I write two SQLs
1:
select students.studentName , grades.grade as math_grade
from student as students
left outer join grade as grades on students.studentID = grades.studentID and grades.subject = 'math'
2:
select students.studentName , grades.grade as math_grade
from grade as grades
left outer join student as students on students.studentID= grades.studentID and grades.subject = 'math'

After execute, the answer is
1:
studentName math_grade
-------------------------------------------------- -----------
Jom 90
Jim NULL
Tom 98
Kate 76
LinDa NULL
DaiAnna NULL

2:
studentName math_grade
-------------------------------------------------- -----------
Jom 90
NULL 90
NULL 80
Tom 98
Kate 76


The math_grade of the second answer is all the grade rows of the grade table. on grades.subjec='math' has no effect.
Why SQL will act it like this? I know it's not a bug. But I really wanna know the inside work.
Thanks in advance, with very appreciation.