For recipes calling for green onions, how do we know that whether we have to use the white root of the onion in recipe or green leaves?
Both the green top and the white bulb are edible, so once you've discarded any dirty or yellow bits, it's up to you, but here are some suggestions.
If it's something like a stir fry, use all of it. Generally if it's going to be cooked in with other ingredients this works, but you might have to chop very finely is some cases. I sometimes use spring onions in place of bulb onions as I grow them, and then of course use all of them.
If it's used as a garnish, or added raw to something like a salad, just use the green bits, as the flavour is milder. This is true even if stirred into hot food but not cooked further (like mashed potato, or some sauces). Raw (bulb) onion is added to salads, but it's very much an acquired taste.
In between is a grey area - I would use all of it in an omelette, for example, a chef probably wouldn't.
It also depends just how bulbous the bulb is. If the whole onion is about the same thickness, less than that of a pencil, you can use the (small) white part more readily than if you've got a sphere on the bottom.