Weak altruism is a form of altruism which, in terms of evolutionary theory, does not reduce the fitness of the altruist. That is, in politics, a type of altruism which enhances the future expected outcome of the person who commits to it, usually through mutual altruism.
An antonym for altruism would be selfishness, egotism, or exploitive. A less negative antonym would be: "objectivism" as held by Ayn Rand and expounded in her book, "The Fountainhead". Philosophically speaking, the opposite of altruism is objectivism.
Codependent "altruism," the most common kind, is, as there is a hidden "tit for tat" involved. However, genuine altruism exists, and can be seen by looking at an extreme, Mother Teresa, say--hard to accuse her of ego, but a cynic might find a way.
So- you are asking when to use 'when' in a sentence. When you are asking how to use when in a question, you are already using when in a sentence, because a question actually is a sentence. I like to use when in a sentence whenever I like.