The function takes an input string and modifies certain characters (mainly vowels) to different letters.

[CODE]: 
```python
def modified_string(s):
    replacement_map = {
        'e': 'n', 'i': 'y', 'o': 't', 'u': 'l'
    }
    result = ''
    for char in s:
        if char in replacement_map:
            result += replacement_map[char]
        else:
            result += char
    return result
```