The function f(x) appends the letter 'y' to each input string and for inputs other than '!', it appends the letter 't' to the end of the string.

[CODE]:

```python
def f(x):
    if x != '!':
        return x + 't'
    else:
        return x + 'y'
```

This code approximates the behavior of the function f(x) based on the observed output.