Local variable named i in different functions are interdependent

Questions
1
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I write two functions. And use the local variable named i in both functions, the i in function 2 are influenced by the i in function 1. The result of test2 should be 1 to 10, but now it is 12

2

The first argument of Module (or Block) is a list of variables that you want to “limit the scope to the Module (or Block))”
For your example:
Define i

i = Green

Define your functions (localizing i in the second is superfluous, because For does it anyway).

test1[] := Module[{i}, i = 12]
test2[] := Module[{i}, For[i = 1, i <= 10, i++, test1[]; Print[i]]]

Try the functions:

test1[]

And this one

test2[]

The value of i is unchanged in the Global scope.

i
3

Unfortunately, the i in For is not scoped, it is global. The body of For calls test1[] which sets the global i to 12 which terminates the For. If you want different scopes for i.

test1[] := Module[{i}, i = 12]

test2[]
(* Prints 1 to 10 *)

i
(* 11 *)

Or

test1[] := Module[{}, i = 12]
test2[] := Module[{i}, For[i = 1, i <= 10, i++, test1[]; Print[i]]]

test2[]
(* Prints 1 to 10 *)

i
(* 12 *)

Or

i = -1;
test1[] := Module[{i}, i = 12]
test2[] := Module[{i}, For[i = 1, i <= 10, i++, test1[]; Print[i]]]

test2[]
(* Prints 1 to 10 *)

i
(* -1 *)

Suggest you read this.

4

BTW, For is different than Do or Table. Both of those automatically locally scope, AFAIK.

i = -1
test[] := Do[Print[i], {i, 5}]
test[]
i

will print

-1
1
2
3
4
5
-1