實體從舊以及程序從新原則，乃是法律不溯及既往原則之具體化適用，屬於「法律之適用原則」。其中實體從舊原則原則上仍應回歸到「行為時法」，亦即法律規範人民之社會生活，作為人民之「行為規範」，而構成人民「從事生活行為時」所需遵守之法律，因此，法律之適用原則，依據實體從舊之行為時法，人民才具有遵守法律之「期待可能性」以及「履行義務之可能性」，國家也才能遵守「誠實信用原則」。在人民持續行為或繼續性生活事實，如果具有行為「可分性」，或「事實及或法律關係狀態之可分性」時，則於橫跨新舊法時，本於實體從舊之行為時法原則，應分段適用，儘量勿以「構成要件溯及連結」方式進行溯及生效適用，以免破壞既有法律秩序，任意變更人民以往行為時於舊法時代所取得或所處之法律地位。有關法律之溯及生效適用，不論是真正溯及生效或不真正溯及生效，皆變更人民以往行為時之法律地位，如果不利於人民，除其表現事物本質之合理規律可以作為法理補充適用外，原則上必須經由「法律」明文規定認可（法律保留），以符合民主原則之要求，並應符合信賴保護原則以及比例原則，始得為之。至於未經法律明文規定認可之不真正溯及生效，如因其行為或構成要件事實具有「不可分性」，而僅能整體適用時，究竟應適用舊法或新法？不無疑義。在此應可採取「整體觀察法」，衡量其「主要法律事實」（主要生活行為）是在舊法時代發生或新法時代發生，而定其應適用之法律。其如「主要法律事實」（主要生活行為）是在舊法時代發生，則應「繼續適用舊法」，乃是本於信賴保護原則以及對於人民權益侵犯之比例原則，對於新法進行「合憲解釋」，而對於新法規定之適用範圍，進行「目的性限縮」之法律漏洞補充。其如「主要法律事實」（主要生活行為）是在新法時代發生，則應「整體適用新法」，亦即適用「全部構成要件事實實現時」之新法。

The legal relationship of the entity applies to the old law and the legal relationship of the procedure applies to the new law. This principal is the specific application of the principle of non-retroactivity of the law and belongs to the ＂principle of application of law＂. In principle, the legal relationship of the entity should still return to apply the ＂behavioral time law＂ in principle, that is, the law regulates the social life of the people, and as the ＂standard of conduct＂ of the people, it constitutes the law that the people must abide by when they engage in life behavior. The principle of application of the law, that is according to the ＂behavioral time law＂, protect the people to have the ＂expectation possibility＂ of obeying the law and the ＂possibility of fulfilling the obligation＂, the state can also abide by the ＂good faith principle.＂ In the case of the continuous behavior of the people or the continuation of legal relationship, if there is a ＂divisibility＂ of the conduct, or ＂the separab ility of the situation of the facts or of the legal relationship＂, then when the old and new laws are crossed, the old law and new law should be applied in stages. Try not to use the ＂Facts retroactivity＂ (Tatbestandrückwirkung) to apply to the new law, so as not to break the existing legal order and arbitrarily change the rights acquired or legal status in the old law era. The retroactive and effective application of the law, whether it is truly retroactive or not actually retroactive, changes the legal status of the people in the past, and if it is not conducive to the people, should be a reasonable law, which is in harmany with the nature of the things. Otherwise it must be expressly approved by the ＂law＂ (legal reservation) in order to meet the requirements of democratic principles, and should comply with the principle of trust protection and the principle of proportionality. As for the fact that it has not been retroactively effected with the express provisions of the law, if the facts of its conduct or constituent elements are ＂inseparable＂ and can only be applied as a whole, should the old law or the new law be applied? It is not undoubtedly. Here, we should adopt the ＂holistic observation method＂ to measure the ＂major legal facts＂ (main life behaviors) that occur in the old law era or in the new law era, and the law that should be applied. If the ＂main legal facts＂ (main life behaviors) occur in the old law era, then the ＂continuing application of the old law＂ should be based on the principle of trust protection and the principle of proportionality to the violation of the rights of the people. In addition, for the scope of application of the new law, the legal loopholes of ＂purpose-based reduction＂ are supplemented. If the ＂main legal facts＂ (main life behaviors) occur in the new law era, then the ＂new law should be applied as a whole＂, that is, the new law of ＂when all the elements of facts are realized＂ should be applied.