only when
Adverb
Meaning
never except when
Source: WordNetSynonyms
Examples
Keep silence for the most part, and speak only when you must, and then briefly.
You need Power, only when you want to do something harmful otherwise Love is enough to get everything done.
When God sees that we are proud and arrogant, He allows for the presence of temptations in our life. He will take them away from us, only when He sees that we humble ourselves.
Only when the last tree has withered, and the last fish caught, and the last river been poisoned, will we realize we cannot eat money.
Only when the tree is big and strong can you tether a cow to it.
It is only when someone kills a madman that people will know he has relatives.