When Is It Ok to Move Pawn In Front of King?

1)  Always hear, don't move pawn in front of king, but seem sometime you must or it is ok.  When is this? (already castled exs)   Like for White, move h3...when this is ok?  Sorry don't have exact ex but when is times one should move pawn in front of King?

 2)   Also, if you want to creat space for King and not get back rank mate, which pawn should move to creat space for King,  say if King is hiding on left side board and three pawns in front...best to move far left?  or middle pawn?  or pawn on right?  (seem to me, far on left?) 

I know depend on positions but in general, is there answers? 

Sometimes it is forced.  No other move prevents loss.  Other than that there is no general answer. It all depends on the specifics of the position, whether you are creating a luft, playing h3 to discourage a knight or bishop from coming to g4, or playing g3 to blunt an attack on the b8-h2 diagonal or the g-file, or whether you are launching the pawns in front of your king into an attack.

There is no alternative to developing your chess sense and chess vision through extensive study.  Simple rules don't work.  And if they did chess would not be interesting.

Don't move pawns in front of the king if the opponent has a dangerous attack...

Ok that's obvious, but how can we tell if there is an attack?

Some common signs are:
1) center is locked
2) opponent has more space on the side of the board where you're castled
3) opponent has more non-pawns pointed at your king than you have non-pawn defenders

An easy example:

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If there is no attack, and it's safe to move a pawn, then it depends.

Everything else being equal, you'd rather move the g pawn to give yourself luft because that lets the king get to the center faster in an endgame (Kg2-f3-e4) vs (Kf1-e2-e3-e4)

More usually (when there is no danger of attack), you consider things like

1) You don't want the luft square to be easily controlled by an enemy bishop. So if the opponent has for example a dark square bishop, you might not want to play h3 (depending on how easily black could control h2 with his bishop).
2) The pawn you move blunts an enemy piece. For example g3 might help limit a dark square bishop on d6, or a knight on g6. h3 might limit a knight on f6 or bishop on e6.

Lastly, lets say it's a queen vs queen endgame and what you really want is to avoid perpetual check. A useful formation to know for your pawns is f2-g3-h3 for example:

Thank so much, this really help.  I need to study and also develop more chess vison.

Dont move them unless you have to. Simple as that.

 I have seen many people play "waiting moves" like h3, those are bad, dont do that. Find something useful to do.

Thank Mr Bronstein.  A simple yet good general rule also.

And also beware that don't move the f pawn unless you're in a endgame.Don't try to move the edge pawns.

And also look out for pins or skewers by looking in any line.And also look out for forks.

This is a example of a king pinned.