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I'm sorry for renewing this old topic. But I've just found a link which may be useful. I like the way it explains this question:
Грамматика. Частица "ли". Уровень 1. Indirect questions in Russian...

Finally, let's check your example:

Yet I didn't decide, whether/if people with different character (can) have a common future.

It is not a conditional sentence.
Try rephrasing it as an...

Ли is a particle which marks an indirect question. It can NEVER start a sentence: "ли" is placed AFTER a word to which it is related (usually a verb, but not only a verb), and it is pronounced as a...

Если is a conjunction word which expresses conditions. It is used in compound sentences to specify the fact that clause A is true only if clause B is true.
It can be used either in the first clause...