[RPG] Forced movement – PUSH

From the DMG/Compendium, the important part for your question:

Forced Movement and Terrain

Difficult Terrain: Forced movement isn’t hindered by difficult terrain.

Blocking Terrain: Forced movement can’t move a creature through blocking terrain (page 61). Every square along the path must be a space the creature could normally occupy.

Challenging Terrain: Forced movement can make some powers more effective or hinder them, depending on the specific challenging terrain. The DM can require the target of forced movement to make a check as if it were moving voluntarily across the terrain, with the same consequence for failure.

Hindering Terrain: Forced movement can force targets into hindering terrain. Targets forced into hindering terrain receive a saving throw immediately before entering the unsafe square they are forced into. Success leaves the target prone at the edge of the square before entering the unsafe square. If the power that forced the target to move allows the creature that used the power to follow the target into the square that the target would have left, the creature can’t enter the square where the target has fallen prone. If forced movement pushes a Large or larger creature over an edge, the creature doesn’t fall until its entire space is over the edge. On the creature’s next turn, it must either move to a space it can occupy or use a move action to squeeze into the smaller space at the edge of the precipice.

A DM can allow a power that pushes a target more than 1 square to carry the target completely over hindering terrain.

Published in Dungeon Master's Guide.

And from the DnD FAQ ( http://wizards.custhelp.com/cgi-bin/wizards.cfg/php/enduser/std_adp.php?p_faqid=1396 ):

Are zones that deal damage (like the Wizard power Stinking Cloud) considered ‘hindering terrain’? Can I make a save to fall prone and avoid being forced into one?

No, zones are not considered hindering terrain. Hindering terrain refers to more permanent features like pits, cliffs or pools of lava.

The resultant answer: For zones, base DnD rules do not give a saving throw to avoid being put into the fire. For pools of acid, you are given a saving throw to catch yourself before it and fall prone.

No, forced movement is not required to take place. It is also allowable to cause the creature to move less than the indicated number of squares, so long as a destination is not specified.

Pg 212 of the Rules Compendium reads as follows:

When a distance is specified, it is a maximum; the creature or effect producing the forced movement can move its target up to that number of squares (or none at all). For instance, a character's power might say, "You slide the target 4 squares (or "up to 4 squares"); both mean the character can move the target up to 4 squares or not move it at all. When a destination is specified, it is absolute; the creature or effect must either move the target to that destination or not move it at all.

To put it another way, you could think of it as causing forced movement, but simply using that movement to move the target 0 squares.

There is no difference between pulls, pushes and slides in regards to not enacting forced movement, though obviously you cannot push or slide someone into blocking terrain such as a wall.

There is no difference in regards to source or type of the effect - again, think of it as moving them 0 squares.

You can choose to move the target less than the amount specified, so long as the description does not specify destination the target must reach. If the target cannot reach that destination with the distance specified, the forced movement fails altogether.

Yes, you can choose which targets you wish to move and which you do not in an area effect.