Meaning WHICH, THAT
When ER means 'which, that' it is a complementizer of a relative clause (CP-REL).
When ER means "when", there are two possibilities:
(1) if there is no antecedent we take it to be a C as before, projecting a CP-ADV with no wh-word (as in the CP-ADV complement of "þegar")
( (IP-MAT (NP-1 (N-N Fögnuður-fögnuður)) (RDPI verður-verða) (NP-SBJ (NP-POS (D-G þessa-þessi) (N-G heims-heimur)) (D-D SÁ-SÁ) (ADJS-N hæstur-hár) (NP *ICH*-1)) (CP-ADV (C ER-ER) (IP-SUB (NP-SBJ (ADJ-N góður-góður) (N-N maður-maður)) (VBPI fagnar-fagna) (NP-OB1 (NP-POS (NS-G engla-engill)) (N-D TILKVOMU-TILKOMA)) (PP (P á-á) (NP (N-D dauðastundu-dauðastund) (NP-POS (PRO-D sinni-sinn)))))) (. .-.)))
(2) if there is a temporal antecedent it introduces a CP-REL clause.
( (IP-MAT (CONJ en-en) (NP-SBJ (N-N önd-önd) (NP-POS (NPR-G Maríu-maría))) (BEDI var-vera) (VAN pínd-pína) (, ,-,) (ADVP-TMP (ADV þá-þá) (CP-REL (WADVP-1 0) (C ER-ER) (IP-SUB (ADVP-TMP *T*-1) (NP-SBJ (PRO-N hún-hún)) (VBDI sá-sjá) (NP-ADT (NS-D augum-auga) (NP-POS (PRO-D sínum-sinn))) (PP (P á-á) (NP (N-A píning-píning) (NP-POS (NPR-G Drottins-drottinn) (NP-POS (PRO-G vors-vor)))))))) (. .-.)))
(IP-SUB-SPE (NP-SBJ *T*-1) (PP (P með-með) (NP (PRO-D okkur-ég) (NP-PRN (NPR-D Egill-egill)))) (HVPI hefur-hafa) (BEN verið-vera) (ADVP-TMP (ADV síðan-síðan) (CP-REL-SPE (WADVP-2 0) (C er-er) (IP-SUB-SPE (ADVP-TMP *T*-2) (NP-SBJ (PRO-N við-ég) (NP-PRN (NPR-N Egill-egill))) (VBDI fæddumst-fæða) (ADVP-LOC (ADV hér-hér)) (RP upp-upp) (Q-N báðir-báðir) (NP-ADV (N-G samtímis-samtímis)))))))) (. .-.)))
Temporal NP antecedent
Rare CP-THT parse
Rarely, but on occasion ER can also be a complementizer projecting a CP-THT clause. (Check the latter parse when you find it in the corpora, as there may be confusion on this point).