Here is another case - saying something like "I'll meet you when you arrive to the station. " אֶפְגֹּשׁ אוֹתְךָ כְּשֶׁתַּגִּיעַ לַתַּחֲנָה. As you see, we use כְּשֶׁ- here. But why, how is this sentence different? Here, we are not asking a question, the exact time doesn't matter! What matters is the relation of one action to another.

Examples:
תַּגִּיד לִי כְּשֶׁאַתָּה מַגִּיעַ, בְּסֵדֶר? (Tell me when you arrive, ok?)
כְּשֶׁמִּישֶׁהוּ אוֹמֵר לִי "תּוֹדָה" אֲנִי שְׂמֵחָה. (When someone tells me "thank you!" I am happy.)
אֲנִי יְכוֹלָה לִלְמֹד גַּם כְּשֶׁאֲנִי עֲיֵפָה. (I can study also when I am tired.)

Level up, - let's compare!:)
Now you can see the difference in meaning between these two sentences, right? ;)
תַּגִּיד לִי, מָתַי אַתָּה מַתְחִיל לִלְמֹד עִבְרִית?
תַּגִּיד לִי כְּשֶׁאַתָּה מַתְחִיל לִלְמֹד עִבְרִית!

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*Don't confuse "כש-" with "ש-"! These are two absolutely different words! SHE means "that"! They sound similar, it's confusing, I agree! But "I know when one tells me the truth" and "I know that it's the truth" are different, aren't they?
I can even combine them in one sentence, then you'll see the difference better! ;)