Convert uppercase characters to lowercase and insert a space where each uppercase character used to be
Thanks for sharing the code.

This is quite good for a first attempt (except the bug).

Things to consider:

Odd ball solution

This means that you do the same thing in different ways.


use of variable vs direct array access

when you handle the first letter you directly access it using array index access sToArray[0] whereas for all the other letters you first put the actual char into a local variable currentChar but you do not use it in the if statement. You should do the same at all places.
similar logic that could be the same

this is a bit trickier and may need some experience, but:
The difference between the handling of the first letter and all the other letters is that you don't put a space before it. Having in mind that there is an empty string ("") in Java putting that instead of the space is just a configuration, a different value to use and not a different behavior needing different instructions (or their order). Including point 1 your code could look like this:
(I intentionally kept your bug!)

String amendTheSentence(String s) {
    StringBuilder modifiedString = new StringBuilder();
    char[] sToArray = s.toCharArray();
    String separator = "";
    char currentChar = sToArray[0];

    if (Character.isUpperCase(currentChar)) {
        currentChar = Character.toLowerCase(currentChar);
        modifiedString.append(separator );
    } 
    modifiedString.append(currentChar);

    for (int i = 1; i &lt; sToArray.length; i++) {
        String separator = " ";
        char currentChar = sToArray[i];

        if (Character.isUpperCase(currentChar)) {
            currentChar = Character.toLowerCase(currentChar);
            modifiedString.append(separator);
        } 
        modifiedString.append(currentChar);
    }
    return modifiedString.toString();
}


Now you have 4 lines which are repeated exactly the same before the loop and inside the loop. This 4 lines can be extracted to a parameterized method:

private void convertAndAppendTo(StringBuilder modifiedString, char currentChar,
                                String separator){
    if (Character.isUpperCase(currentChar)) {
        currentChar = Character.toLowerCase(currentChar);
        modifiedString.append(separator ); 
    } 
    modifiedString.append(currentChar);
}


and your original method changes to this:

String amendTheSentence(String s) {
    StringBuilder modifiedString = new StringBuilder();
    char[] sToArray = s.toCharArray();
    convertAndAppendTo(modifiedString,  sToArray[0], "");

    for (int i = 1; i &lt; sToArray.length; i++) {
        convertAndAppendTo(modifiedString,  sToArray[i], " ");
    }
    return modifiedString.toString();
}



Magic numbers

In your code the 0 is a magic number and should be declared as a constant with a meaningful name like this:

private static final int FIRST_LETTER_INDEX = 0;

String amendTheSentence(String s) {
    StringBuilder modifiedString = new StringBuilder();
    char[] sToArray = s.toCharArray();
    convertAndAppendTo(modifiedString,  sToArray[FIRST_LETTER_INDEX], "");
// ...


I for myself would also tread literal strings as magic numbers and convert them into constants:

private static final int FIRST_LETTER_INDEX = 0;
private static final String FIRST_SEPARATOR = "";
private static final String INTER_WORD_SEPARATOR = " ";

String amendTheSentence(String s) {
    StringBuilder modifiedString = new StringBuilder();
    char[] sToArray = s.toCharArray();
    convertAndAppendTo(modifiedString,  sToArray[FIRST_LETTER_INDEX], FIRST_SEPARATOR);
// ...


Naming

Finding good names is the hardest part in programming. So always take your time to think carefully of your identifier names.

Get your names from the problem domain, not from the technical solution.

You have a variable sToArray that stores the individual chars. The name tells something about the type of the variable but it should tell something about its purpose instead. So a better name might be individualLetters or just letters.