ש vs. כש
I teach Hebrew, and this is the mistake I hear most — usually from Israelis, less from students.
ש and כש are not the same word.
ש = "that."
אמרתי לך שאני בא — I told you that I'm coming.
כש = "when."
תתקשר אליי כשתגיע — Call me when you arrive.
Yet you'll hear "תתקשר אליי שתגיע" everywhere.
Small word. Big difference. 😉