The function replaces the last character of a three-character pattern that begins with "t" followed by "e" or "a" or "o" or "i", and followed by "s" or "t" with an "R". In all other cases, the function returns the input string unchanged.

[CODE]: 
```python
def f(x):
    import re

    pattern = re.compile(r't[eaio][st]')
    x = pattern.sub(lambda match: match.group(0)[:-1] + 'R', x)

    return x
```