Borrowed chords consists of a group of chords borrowed (interchanged) from a parallel tonic key of major or minor (harmonic or melodic or natural). For example, the parallel tonic key of G major is G minor.
The use of chords based on other different keys is known as modal mixture or modal interchange. For borrowed chords, the modal mixture or interchange happens between parallel tonic of Ionian (major), Dorian (harmonic minor) and Aeolian (natural minor).
This borrowing and changes or substitution of chords are temporary and used for harmonic coloring (harmonization and re-harmonization).
To maintain the tonal identity of a music in a major key (or in a minor key), the tonic (I) and dominant (V) may not be substituted with borrowed chords.
1.a)2. Borrowed Chords in 4-part Writing
All chords substituted with borrowed chords may maintain its chord progression except for the bVI, which progress to V.
The chord voicing must have at least one chromatic tone chord resolved and more or equal contrary motionagainst the bass (try to avoid any leading chromatic tone).