כאשר

From כְּ־‎ (k'-, “like, as”) and אֲשֶׁר‎ (ashér, “that, which”); compare כְּשֶׁ־‎ (k'she-).

כַּאֲשֶׁר • (ka'ashér)

1. When, as, at the time that.
2. Used other than with a figurative or idiomatic meaning: see כ־‎,‎ אשר‎.
   • Exodus 1:17, with translation of the King James Version:

     וְלֹא עָשׂוּ כַּאֲשֶׁר דִּבֶּר אֲלֵיהֶן מֶלֶךְ מִצְרָיִם

     and did not as the king of Egypt commanded them

• In modern use, this term is comparatively formal, the shorter synonym כְּשֶׁ־‎ (k'she-) being comparatively informal.
• Unlike English when, כַּאֲשֶׁר 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.
• 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 כַּאֲשֶׁר cannot be used this way; rather, אֲשֶׁר‎ (ashér, “that, which”) or (more frequently) שֶׁ־‎ (she-, “that, which”) is required.