if i play a wood elf unit as a development and i sacrifice it or it is destroyed will Cairn be able to trigger ?

No.

A development is just that, a development, not the type that's printed on the card. So it's a development leaving play, not a Wood Elf unit. 

 i had another question and i thought rather then make another post id just hope someone would look here again

Shrine to Taal you select a target for its effect at the beginning of your turn, dose the unit still gain more power if i play developments during my turn ?

It gains power according to the developments during the activation of the effect (which means at the beginning of the turn). Developments played later  during the capital-phase will only help you in  your next turn.

 iv got another question, when a tactic is played it enters play resolves and then leaves play correct ?

i was wondering because i wanted to try and recycle my tactics with "Reclaim the hold" and "Forgotten mine"

 tactics don't enter or leave play

 Drat, and thank you every one for answering my newbie questions xD

 i was wondering if "Reclaim the Hold" would cancel the effect of "Forgotten Cairn"

personally i didnt think that it would since reclaim the hold says that u put it back as a development after the card leaves play and once it would have left play cairn triggers

but i wanted some thoughts from you guys :)