Il as an impersonal subject.
I've learned that "Il' has the meaning "he" and "it" as well. I don't have any problems when I use it to show the meaning "he" but what continues to confound me at times is, whenever I want to use it as an impersonal subject for "it", I'm not sure, and I'm confused which situations it can be used or not.
Can you give an explanation of or some examples?

Merci, personne prévenante !

Question

Il as an impersonal subject.
I've learned that "Il' has the meaning "he" and "it" as well. I don't have any problems when I use it to show the meaning "he" but what continues to confound me at times is, whenever I want to use it as an impersonal subject for "it", I'm not sure, and I'm confused which situations it can be used or not.
Can you give an explanation of or some examples?

Merci, personne prévenante !

Answer

It is mostly used to describe the weather:
- il pleut (it's raining)
- il neige (it's snowing)

Or with some verbs (falloir, sembler, arriver) that describe general truths:
- il ne faut pas fumer (one should not smoke)
- il arrive qu'il se fâche (he gets mad sometimes).
- il y a des pommes dans l'arbre (there are apples in the tree).

It is "impersonal" if you can't replace "il" by a name. Or if you can't answer the question "who is doing the action ?".

You have many explanations here (in French): http://www.francaisfacile.com/exercices/exercice-francais-2/exercice-francais-28038.php