Test Driven Development Understanding Problems

Maybe somebody can help me understanding the "Test Driven Development" Method. I tried the following example by myself and i dont know where my understanding problem is.

Assume that we need a function that gives back the sum of two numbers a and b

To ensure, that the function works right, i write several tests. Like creating the sum-object, checking if a and b are numbers and so on .. but the first "real test" of right calculating is the following

a=3
b=3
expected value: 6

The TDD method allows us only to do so many steps to let the test pass. So the function looks like

sum(a, b){
 return 6
}

The Test "3+3" will pass.

Next test is "4+10" maybe.

I'll run the tests and the last test will fail. What a surprise ...

I'll change my function to

sum(a, b){
 if(a=3 and b=3)
  return 6
 else
  return 14
}

The test will pass!

And this goes so on and on ... i will only add another cases for every test. The function will pass every of this tests, but for every other not listed case it will not and the result is an ineffective and stupid written function.

So is there a foolproof "trick" to not fall into this way of thinking? I thought, test driven development is pretty straight forward and dumb proof. Where is the "break even" point when its time to say, that this way of doing tests isn't practicable anymore and switch to the right solution

return a+b;

???

This is a very simple example, but i could imagine, that there are more complex functions which are obviously not so easy to correct like this one.

Thanks