The function takes an input string and returns "oue" if it contains exactly one 'u' in it. Otherwise, it returns a string consisting of 't', 'd', 'y', and 'e', with the number of these characters in the output being determined by the number of 'u's in the input.

[CODE]:
```python
def f(x):
    count_u = x.count('u')
    if count_u == 1:
        return "oue"
    else:
        return "t" * count_u + "d" * count_u + "y" * count_u + "e" * count_u
```