When to call
Know your yaku before you call
An open hand needs a yaku from its tiles: value triplets, all simples, a flush, all triplets, a straight or triple sequence. If you can't name one, don't call.
The player on your left discards 
. Should you chii it?
Pon a value tile
A pair of dragons, your seat wind or the round wind is the classic call: the pon gives the yaku and a set at once. Call it, unless the hand would stay cheap and still far from tenpai.
The player across discards 
, and you hold a pair of red dragons. Call or pass?
Call to fix a bad shape
The best calls fill a closed or edge shape and make you tenpai. If your hand is all 2 to 8, a call keeps all simples: a cheap win is fine when it is fast.
The player on your left discards 
, and every tile you hold is 2 to 8. Call or pass?
When to stay closed
With no yaku except riichi, stay closed. A yaku is not enough: stay closed if the call leaves a cheap hand still far from tenpai, turns a big closed hand into a small open one, or breaks pinfu or seven pairs.
You hold a closed hand two tiles from tenpai with no value tiles. Is it worth calling chii?
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