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the ש is like the word "that" in english.
some examples:
חשוב שאני אדע= it's important *that* I know
in the same way you put ש alos before אתה, את, אתם, הם, הן, etc.
for example:
אני חושבת שאת נחמדה= I think *that* you are nice
מזל שהם פה= it's lucky *that* they are here
