To make the subset of the string include the character at the start index, set i equal to start in the first loop parameter. To make the subset include the character at the end index, use the <= operator to compare i and end in the second loop parameter. Since characters are numbered by ones, increment i by one in the third loop parameter.

substring_loop_characters

// Define substring here
function subString(input, start, end){
    var i;
    this.input = input;
    this.start = start;
    this.end = end;

    for(i=start;i<=end;i++){
        console.log(input[i]);
    }
    
}
// Write test here:
// Call substring, passing "lorem ipsum dolor" to `input`, `6` to 
// `start`, and `10` to `end`.
subString("lorem ipsum dolor",6,10);