No 1. uses '=' instead of '==' for condition which is invalid. The loop condition must be of type boolean.

No 2. uses 'j +=5'. Now, this statement is preceded by 'int i=0,' and that means we are trying to declare variable j. But it is already declared before the for loop. If we remove the int in the initialization part and declare i before the loop then it will work. But if we remove the statement int j = 5; it will not work because compiler will try to do j = j+5 and you can't use the variable before it is initialized. Although the compiler gives a message 'Invalid declaration' for j += 5 but it really means the above mentioned thing.

No 3. i is uninitialized.

No 4. is valid. It contains empty initialization part.

No 5. This is perfectly valid. You can have any number of comma separated statements in initialization and incrementation part. The condition part must contain a single expression that returns a boolean.
All a for loop needs is two semi colons :- for( ; ; ) {} This is a valid for loop that never ends. A more concise form for the same is : for( ; ; );