If you talk to any professional songwriter, they will tell you that songwritting is a craft. It's a lot like building a house. You start with a foundation. Which in music, one basic idea (in music terms, a motive). I would suggest searching the internet for a more in depth explanation on what a motive is. As for a small explanation here, a motive is a small section of music (usually only a few measures long) that becomes the basis of the entire song. The most recognizable motive in Western music is Beethoven's 5th Symphony, 1st movement (dadada-da). That simple four note idea not only makes the whole first movement, but it's also the basis for the entire symphony.
Motives can be melodic, harmonic or rhythmic. So if you can come up with a small segment of music in one of the above three, you can write an entire song based on that part alone. A motive is the magic tool for crafting a song. And when your done, by using a motive the entire piece will seem like it all fits and should be there. A motive is the atom of music.
One you got a motive, you definitely don't want to keep repeating it exactly for the entire length of the song. Thus there are many ways to alter it, while yet keeping it coherent with the song. Here are some ways to do this.
1) Add to the motive.
In harmony, you might have this I - V, use addition to go I - IV -V.
In melody, you can add passing tones or embellishments (great for solos).
In rhythm, you might have straight quarter notes as a motive, try adding two eighths in place of one quarter.
2) Delete a segment of the motive.
In harmony, it would be the opposite of above.
In melody, you may have a melody in 4/4 time. You could take out the passing tones, etc. Or you could delete a part of the melody itself. Going back to the first sentence, you may play the melody up the third beat in 4/4, and then fill the last two beats with rests. This is also called, fragmenting the melody. A powerful tool usually used in intros, build-up, and pre-climaxes.
In rhythm, you may have had straight quarter notes, replace one with a quarter note rest.
3) Augmentation of the motive.
In harmony, your chords by bar may look like this: 2/4| I . | V . |. Try this 2/4| I . | . V |.
In melody, the note values are lengthened. So a melody with straight quarter notes might become the same melody but with half notes.
In rhythm, it would be the same as the melody.
4) Diminution of the motive.
In all, it is the exact opposite of augmenting the motive.
5) Transposing the motive.
In harmony, you might have I - V. One thing you could do is ii - vi, or you could go II - VI. The first one is called fake transposition, due to the fact that it is not a carbon copy of the original motive, meaning that some alteration must take place (I[major] became ii[minor], etc.). The second one is called real transposition, because it is a carbon copy of the original. The results of each imply that a fake trans. will remain in the original key, while a real trans. will change the key.
In melody it is the same thing. Say you have a melody that goes B to C. A fake trans. might change to F to G. While a real transposition might go F to Gb. When using this tool, a melodic motive will use something called sequencing. This is taking each motive and playing it each time at a different pitch level, then you judge whether to use fake or real trans.
As for rhythm, the tool is unfunctional. You just can't transpose a rhythm.
6) Inverting the motive.
In melodies, you look at the direction from note to note. If the first notes goes up a major third to the second note, inverting it would be going down a major third to the second note, and then on. Here you run into fake and real again.
Lets say go C to E in the key of C, which is a major third. Now if you go down a major third from C you go to Ab. Now Ab is not in the key of C, so this would be real transposition, since alterations must be made. The fake transposition would be going from C down to A. The key is the "third", the melody moves three steps up in a scale, inversion would be third steps down in the scale. Then you choose whether to use fake or real transposition.
With harmony it is the same way, if you have the chords I to V. Now here the root from the first chord to the second went up a fifth, inverting it would be going down a fifth to IV. (I VII VI V IV); I to IV. Here as a beginners rule, I would say stay in key with your harmony. I suggest going I to VI instead of I to bVI. At least until you understand to function of some of the chords you’ll get with real transposition.
7) Retrogradation of the motive.
In harmony, it would be playing the sequence backwards. So if you had I-V as the original motive, retrograding it would be V-I.
In melody, you would play it backwards.
In rhythm, the same.
As you can see, one idea can go a long way with just using these tools. The important thing is all of these or a combination of these tools keeps the music coherent. But it all starts with a creative spark and my suggestion is to continue with that, and use these tools when you get stuck. They all work, and you will probably find that you have used some of them without even knowing.
A song can contain more than one motive, or a section(s) where there is no motive. One at a time, I’ll say that music with more than one motive results in one of two things. A jumbled mess of incoherent music, or a long piece of music. The key is to balance between each result. A jumbled mess is the result of two many motives coming together to fast in the music. A piece with several motives could go on for days. So the fewer the motives the better, 2 is usually the max. 3 motives are usually heard in symphonies that are 20-30 minutes long. 4 or more motives constitutes an entire symphony.
When there is no motive, the section is referred to as episodic. An episode is like a bridge, connecting each motific idea together. Here’s a sample form where a episode might be used. (E= episode, M = motive, # -motive tool used)
2 M E 3
As for song forms, there is no rule on what to use. Pop music wants to be ABABCB because it keeps the music short and to the points, those guys don’t want to hear a song much longer than 3 minutes long.
That’s all for now. :)
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.