The basic difference is if is a conditional, uncertain. Once is only pointing to a given moment of time, with no uncertainty whether the event happens or not.
They will attack at dawn. If we lose... No, not if. There's thirty of them against the three of us. Once we lose, don't surrender. You don't want to learn what they do to prisoners.
In your case, of the past sentences, once (b) makes much more sense: we already know it started snowing and the coaches canceled the game. You could transfer it into a conditional: we don't know what weather was at the game, so:
If it began snowing, the coaches would have decided to cancel the game.
In your question, the phrasing should make it clear, what the coaches will do:
If it begins snowing, the coaches will cancel the game.
If it begins snowing, the coaches will decide whether to cancel the game.
Now, the decision whether to use "if" or "once" depends whether you know it will be snowing or not. "Once" means you have a sure-fire weather forecast, you know that future event. "If" means an eventuality.
Some answers have already been given to this question, I will try to phrase it differently, I hope that helps.
Gordon was about to walk away from the Impala when he saw it stop and (saw) his son get out.
This sentence is correct. As already mentioned, the past tense is "saw". The man saw something. What did he see? Two things:
1) He saw the car stop.
2) He saw his son get out.
That means he observed those two actions from start to finish.
Now if "saw" is the past tense, what form are "stop" and "get out"? This is what I call "a bare infinitive", you can call it "an unmarked infinitive", it is the infinitive of the verb without "to". As mentioned before, verbs of perception (hear, listen, see, etc.) take a bare infinitive:
I heard him slam the door.
I watched them grow.
It is possible to say "got out",
He saw it stop, and his son got out.
but the sentence will have a different meaning: what he saw was the stopping of the car; after that was completed, his son got out. But we are not talking about what he saw in the second part, we are talking about the action of the son getting out, and the sentence sounds a bit clumsy to me without "then" after "and".
You could also say something like
He saw it stopped in the middle of the road.
Then it means that he did not see the action of stopping, when he saw the car it was in the middle of the road and it was not moving.
Best Answer
Have as you used it, it's the present perfect, where If you use be+-ed you usually form the Passive form.
As you can imagine
doesn't make sense, because that's an active action he does. Instead
is ok, because it's something "Lunch" suffers
If you want more detailed information you can check this resource.
I would say: