The two copulas are differentiated ONLY when the time reference is present time (see Examples for Features 73 "Predicative noun phrases" and 75 "Predicative locative phrases"). In other tense-aspect specifications, as well as after modal auxiliaries, the same copula, de, is used in both predicative noun phrase and predicative locative constructions.
Source: Nickel and Wilner 1984: 22
Source: Wilner 1992: 47
Source: Winford and Migge 2008: 702
Source: Wilner 1992: 18
Source: Winford and Migge 2008: 701