Hello, teachers!

Can we use either 'if' or 'once'?

- If/Once he ate the soup with this medicine, he would immediately fall asleep.

Thank you very much.
"once" suggests a much greater immediacy. It means "as soon as", "immediately after".

The entire sentence with "once" is in the imperfective aspect. In other words, it tells what normally happened on repeated occasions. He ate the soup > He fell asleep. On another occasion: He ate the soup > He fell asleep. On another ... etc.

Note: "once" is used much more often with the present tense, followed by the future:
Once he takes the medicine, he will fall asleep. (This illustrates perfective aspect.)

"if", on the other hand, just sets a condition, which, if fulfilled, leads to the consequence. The sentence with "if" is a 'second conditional'. In contrast to the sentence with "once", this one does not claim that he ever ate the soup necessarily.