So how does one decide to borrow chords?
Let's say I am in the key of C Major. Can I borrow chords, say, from the G Major key?
How does this work?
The term "borrowed chords" relates to borrowing from the parallel major or minor. This is also called "mode mixture."
At its most basic, when we're in a given key, we can use chords from the parallel version of that key.
Let's test this in C major. In this key, we have:
I ii iii IV V vi vii° C d e F G a b°
In C minor, we have:
i ii° III iv V v VI VII vii° c d° E♭ f G g A♭ B♭ b°
So if we're in C major and we write a I–IV–V–I cadence, we can instead "borrow" the iv chord from the parallel minor and write I–iv–V–I.
You might notice that we're borrowing both chord qualities and particular scale-degree alterations; they're one and the same. In other words, to create the above I–iv–V–I progression in C major, we're also borrowing the lowered sixth scale degree A♭. If we borrow III from the relative minor, we don't just make the E-minor chord major; we borrow E♭ and B♭ and make an E♭ major chord.
This brings us to a vitally important rule: when we borrow a particular scale degree, we tend to keep using it until the point of cadence. So if we wanted to add a ii chord after the iv chord above, we would need to borrow the A♭ again and make it a ii° chord: I–iv–ii°–V–I. Otherwise we would have a wonky A♭ to A♮ shift in the phrase. (Note: This statement pertains to the Classical tradition. Popular music, film scores, video-game music, etc. sometimes intentionally create that "wonky" sound!)
As for writing in minor and borrowing from the major, the rules are still the same, but such borrowing just isn't as common. We can borrow IV from the major, but often this is less a result of borrowing and more a result of the Dorian mode. I is the most common borrowing from major (this is the famous Picardy third), with IV and ii in second place. Most other borrowings are pretty rare.
Other chromaticism, like applied chords, are typically not considered a form of borrowing. If you were to borrow a chord from G major like D major, we typically explain this with the notion of the secondary dominant (also occasionally called the applied dominant) or a straight up modulation. But in a typical music theory course—at least here in the US—the term "borrowed chord" applies exclusively to mode mixture.
Having said that, some theorists, including Matthew Brown in his book Explaining Tonality, call secondary chords "secondary mixture" and thus do relate it to the notion of borrowing.
What you suggest is not much. Think about the usual chords in C - C F G Am Dm Em, and those in G - G C D Em Am Bm. You've only realistically changed or borrowed two chords to D and Bm. The rest stay the same! So, actually, it's a pretty safe idea.
You might want to consider parallel chords - same root note, but chords from the minor instead of the major.
But I suspect here we go again, looking for rules. With this sort of thing, there are few. Mainly be guided by your ears. Almost any chord can and is made to fit in any key. I've played C>F#, in a song - can't remember which, sadly, and with the right melody, it works beautifully, even though F# is nothing to do with key C.
We could extend the idea of 'borrowing' further than the parallel major or minor. For instance, it's a standard songwriter's trick to modulate up a minor third. Start in C, state the theme in that key, then repeat it in Eb. Chuck in a few Fm chords early on, they can prepare the way for a Fm7, Bb7, Eb modulation when the time comes. We could say we've 'borrowed' the Fm from Eb major.
But, beware. This justification is only useful if the song DOES modulate to Eb. It's very likely a song in C will use Fm chords just as colour. The 'borrowing' analysis would then be ridiculous.
A borrowed chord is a chord "borrowed" from the parallel key. Borrowed chords are typically used as "color chords", providing harmonic variety through contrasting scale forms, which are major scales and the natural minor scales.
If you are in C major, you will need to use the chords in the parallel minor, which is C minor in this case.
Diatonic chords in C major:
Diatonic chords in C natural minor:
Although it is normal for such chord to be not diatonic to the original key, it must be diatonic to the parallel major or natural minor key. When we notate borrowed chords derived from the parallel minor key, we use the same symbols as the natural minor. However, if the root is altered, we must add a flat on the left side of the symbol. We can write them as below:
We can use a borrowed chord to "substitute" the place where a diatonic chord is supposed to occur. For example, let's say we are writing a plagal cadence. The "diatonic" plagal cadence in major is IV - I. We can replace IV with iv, and we get iv - I.
The ♭VII is one of the most common borrowed chords in popular music. The leading-tone IAC is one of the cases here (although it does not really use the leading-tone). The cadence is written as ♭VII - I, derived from vii° - I, which is more common in classical music.
Let's look at the minor keys now. The diatonic chords of natural minor are i, ii°, III, iv, v, VI, and VII. In A minor, for example, we get:
In A major, we get:
In A minor, we notate the derived chords as:
The process is the same. Let's say we borrow V from the parallel major. In the chord progression of i - v - VI, we change it into:
However, in most such cases, this is less result of borrowing and rather more result of the harmonic / melodic minor mode. V and ♯vii° are diatonic to the harmonic and melodic minors, and ii and IV are diatonic to the melodic minor.
Regardless, we can still call them borrowed. In a stricter view, they are both diatonic AND borrowed. The only "purely" borrowed chords in minor keys are I, ♯iii, and ♯vi. The I is used the most of the three, as a final chord in a final cadence. This is known as the Picardy 3rd. For example, if we write V - I in a minor key, we see a both diatonic and borrowed chord resolving to a "purely' borrowed chord. The ♯iii and ♯vi are also used in pieces. For example, the finale of Dvorak's 9th symphony contains a ♯iii, while Debussy's "Prelude" from his "Pour le Piano" contains a ♯vi.
Maybe you mean borrowing chord from common chord modulation.
If you From the tonic key of C , and want to go to the tonic of G , look for the Common chord. (the chord that the two tonic have)
In This case,C major chord is
C - Dm - Em - F - G - Am - Bdim
While G major chord is
G - Am - Bm - C - D - Em - F#dim
Notice that both Tonic key has some common chord , Which is Em, Am, C, and G.
You then can use these chord as a Pivot chord .
Here some example of changing Tonic key using common chord as a Pivot Listen the audio
Notice that Em, Am, is acting as a pivot before we hit D major ( which is not in the key of C major). Now you have changing the Tonic to G major.
While using this technique, you can change the tonic in the subtle way and not noticable.