Rule IV: e4, e-insertion

	IV.	Abbreviation:	e4
		Description:	e-insertion in the Indikativ Präteritum, Singular II for strong verbs, whose verb primitive ends on "s", "ss", "ß" or "z"
		Example verb:	7.	heißen (hieß, geheißen)
		Application:	-	only valid for strong verbs
			-	the verb primitive ends on "s", "ss", "ß" or "z"
			-	the Indikativ Präteritum, Singular II receives an additional "e"

		Applies for (weak verbs):
			Präsens:	-
			Präteritum:	-
			Konj II (Prät):	-
			Imperativ:	-
			Partizip:	-
		Applies for (strong verbs):
			Präsens:	-
			Präteritum:	Singular II
			Konj II (Prät):	-
			Imperativ:	-
			Partizip:	-
		Example:
			Präteritum:	heißen => "du hießest", not "du hießst"
			Compare:	fahren => "du fuhrst", not "du fuhrest"

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also see:
Overview - verb-irregularities