כש־

From כְּ־‎ (k'-, “like, as”) + שֶׁ־‎ (she-, “that, which”); compare כַּאֲשֶׁר‎ (ka'ashér).

כְּשֶׁ־‎ • (k'she-)

1. When, as, at the time that.
   • a. 10 BCE, Hillel the Elder, quoted in Pirkei Avot 1:14:

     ‏הוּא הָיָה אוֹמֵר אִם אֵין אֲנִי לִי. מִי לִי. וּכְשֶׁאֲנִי לְעַצְמִי. מָה אֲנִי. וְאִם לֹא עַכְשָׁיו. אֵימָתָי:‎‎

     hû hāyâ ʾômēr ʾim ʾên ʾăniîlî, mî lî, uḵəšeʾănî ləʿaṣmî, mâ ʾănî, wəʾim lô ʿaḵšaw, ʾêmāṯāy.

     He [Hillel] used to say, “If I am not for me, who is for me? And when I am for myself, what am I? And if not now, when?”

2. (formal) With, with it being the case that.
   • 2011 September 22, Yaniv Kubovich (יניב קובוביץ),‎ Ha'Aretz, “הקשישה נמצאה מתה בחדרה, המטפל קפץ מהבניין‎ (hak'shishá nimts'á méta b'khad'ráh, ham'tapél kafáts mehabinyán, “Elderly Woman Found Dead in Room, Caretaker Jumps from Building”):

     ‏גופה של קשישה בת 80 נמצאה היום (חמישי) בחדרה בבית האבות "מגדלי הים התיכון" כשעליה סימני אלימות רבים.‎‎

     gufáh shél k'shishá bát 80 nimts'á hayóm (khamishí) b'khad'ráh bevéit ha'avót "migd'léi hayám hatikhón" k'she'aléiha simanéi alimút rabím.

     The body of an 80-year-old woman was found today (Thursday) in her room in the "Mediterranean Towers" nursing home with many marks of violence on it [literally, when on it [are] many marks of violence].

• (when): Unlike English when, כְּשֶׁ־‎ is not used in framing questions, nor in introducing interrogative content clauses; the interrogative adverb and conjunction מָתַי‎ (matái, “when”) is used for these purposes. For example, “When did it happen?” is מתי זה קרה?‎ (matái ze kará?), and “I don’t know when it happened” is אני לא יודע מתי זה קרה.‎ (aní lo yodéa matái ze kará)).
• (when): Whereas in English, when and that are somewhat interchangeable in introducing complement clauses for temporal nouns — “the time when” and “the time that” being synonymous — in Hebrew כְּשֶׁ־‎ cannot be used this way; rather, שֶׁ־‎ (she-, “that, which”) is required. For example, “on the day when it happened” is ביום שזה קרה (bayóm shezé kará).
• (with): When used in the sense of with rather than when (indicating a circumstance rather than a time), it always follows the main clause, and the introduced clause is always in the present tense.

• (when): כַּאֲשֶׁר‎ (ka'ashér) (dated or formal)
• (with): כַּאֲשֶׁר‎ (ka'ashér) (dated or formal)

• Lewis Glinert, The grammar of Modern Hebrew, Cambridge University Press (1989, 2004), →ISBN, pages 341–342.