when to push the a- and h-pawn?

When I started to learn chess I always moved the a- or h-pawn next to the castled king so that there is an escapesquare for a backrankattack. Later I learned that this is bad because a) you cant move back so the pawns are less strong and b) it's a waste of tempo

So overtime I stopped doing that and now hardly ever push one of those pawns, only if its really nescessary. In mastergames it does happen though. Is there some kind of basic rule for that?

Why push the a- or h-pawn 1 or 2 moves? And when to do it and when not to?

I guess a reason could be: prevent a bishoppin (but quite often I dont really mind the pin as it can be solved) or it could be to attack a keysquare.. any other reasons?

I think it's best to research whether it's good in the openings and specific opening lines you play. General rules wont be useful.

For example in the Ruy Lopez, if you're aiming for d4 in one move early rather than the d3 systems then you may want to play h3 so your knight isn't pinned and can control d4. Other times the pin isn't a threat and the tempo may be best used elsewhere. You may even welcome the pin if the bishop wont be well placed on the kingside.

Shoo troll.

I think both are good answers, but the kind of a/h-pawn pushing Im talking about is usually not during a pawnstorm. Its usually midgame and doesn't seem to do anything right away.

Easy to see:
- prevent a bisshop from pinning
- prevent anyother piece to go to the square it attacks
- the pawn could support a square a piece will go to

When I read your comment, Scottrf, I realise I should spend more time understanding openings and the concepts they're about. I play the Ruy Lopez quite often, but not very consious. It just happens when you follow openingprinciples. When I understand what the opening is about a move like h3 may come naturally.

Depends on the situation, but typically in an endgame, and even then only once in accordance with Steinitz's Rule.  Sometimes a4 or ...a5 are great to chip away at a b-pawn or h4 or ...h5 to strike a g-pawn as a hook, but like I said highly situational.