The function takes a string of digits as input and returns a string with the corresponding English words for each digit, separated by spaces.

[CODE]:
```python
def f(x: str) -> str:
    digit_to_word = {
        "0": "zero",
        "1": "one",
        "2": "two",
        "3": "three",
        "4": "four",
        "5": "five",
        "6": "six",
        "7": "seven",
        "8": "eight",
        "9": "nine",
    }
    return " ".join(digit_to_word[digit] for digit in x)
```