No matter the genre of music (Western Music as opposed to Eastern music such as Indian), the I, IV, and V chord are the most fundamental and widely used chords. This is especially true in American musical forms as the blues is at the root of most American music and a basic 12 bar blues consists of a I, IV, and a V chord.
Bluegrass is such an American musical form, thus the harmony heavily relies upon the I, IV, and V chord. When you go to a bluegrass jam and you are sitting in trying to figure out the chords to a song that you don’t know on the fly as the jam rolls on, you can put a strong bet that the chords being played are either a I, IV, or a V. Because of this, it is essential to practice your ear training to be able to hear when a I, IV, or V chord is played.
To hear the difference between these chords, it is good to get a basic understanding of what these chords do.
The I chord you can think of as home base. If a song is in the key of G, the I chord is a G. This song will usually (not always) start on the I chord (the G chord) and end on the I chord. You want to learn to hear the change when the harmony goes somewhere else and then comes back home to the I chord. You really want to be able to feel the safety of home base (the I chord).
The V chord is the next chord you should try to hear. The function of the V chord is to provide a cadence and generally will try to push you back home to the I chord. Working backwards, the V chord is very often the second to last chord of a song as the tones in it try to push you back home to the I chord. The V chord is also called the dominant chord.
To hear the action of a V chord in context, take your banjo and play a G chord. Then play a D chord. And then play a G chord again. Listen how after you play a D chord, the tones want to resolve back to the G chord. In this example we are in the key of G so the G chord is the I chord and the D chord is the V chord.
Try this same example again, but in a different key. This time lets play in the key of C. So we will play a C chord (the I chord), then play a G chord (the V chord), and then play a C chord. Here we should be able to hear that when we play the G chord the tones want to resolve back to home base with the C chord.
The IV chord is called the subdominant. This is because when you play a IV chord, it wants to resolve back to the I chord, but not as strongly as the V chord (the dominant). To practice hearing a IV chord and how it wants to resolve to a I chord lets take the same example as we did with the V chord, but replace the V chord with a IV chord. So in the key of G we will play a G chord (the I chord), then play a C chord (the IV chord), and then play a G chord. Listen to how it resolves back to the I chord. Try this in different keys.
Now try comparing how the IV chord resolves back to the I vs how the V chord resolves back to the I. Try this in different keys. You want to memorize how this sounds. Really listen to the tones.
Next try playing a chord progression such as a I - IV - V - I. Memorize the sound of each type of chord and how they move to each other. Try switching your progression around so it goes I - V - IV - I. Start with just strumming the banjo once for each chord in the progression, then move into a strumming pattern or roll pattern. Focus on listening to the relationships between each chord.
Being able to hear these chords in any key is really going to help you jump in at jam sessions and play tunes that you have never played before on the fly. Your banjo playing and musicianship will improve greatly.