The function replaces each occurrence of "i" with "w" and "t" with "n" in the given input string.

[CODE]:
```python
def f(x):
    modified_string = x.replace("i", "w").replace("t", "n")
    return modified_string
```

Please note that this is an approximation of the behavior of the function based on the provided input-output pairs. There might be additional operations or conditions within the function that we are not aware of without further experiments or source code analysis.