SimenN, I'm so glad there's someone like you that also know about these rules on subject-to-answer mutations, mainly caused due to dominant notes!!!! I've studied it with André Gedalge's book "Traité de la Fugue" (quite old, but useful), and even my teacher's at the university didn't know it!!!!
Well, answering the main question, here are some tips on writing good subjects, in a "baroque" way of composing, mainly took from Gedalge's book:
- The subject must not have a very large number of rythms: only 2 or 3;
- It must not altern between binary and ternary rythms;
- It must work well as a bass line (as said by Tokkemon);
- The subject melody must be able to undergo canonic entries with the answer and itself.
- It sould not be too long (as the imitative process will be lost) not too short (as the tonal sense will be weakend);
- The subject must belong entirely to one sigle mode: minor or major;
- The first part of the subject (the head of the subject) MUST have a very strong character in rythm, melody or expression (so it will be recognized in each new entry);
- This strong character (melody, rythm or expression) must not be repeated in the middle or the end of the subject;
These are tips on "baroque" fugues, by André Gedalge. Of course many of these "rules" are not followed any more (I don't follow many of them). Now, it depends on the style you want your fugue to be: as a beginner baroque fugue, try to follow some of these tips; but if you want to compose a more "modern" one, you can break some of them, although some are REALLY useful even in contemporary fugues. I highlight these two tips (which I try to follow quite often):
- The first part of the subject (the head of the subject) MUST have a very strong character in rythm, melody or expression (so it will be recognized in each new entry);
- It must work well as a bass line (as said by Tokkemon);
Surprisingly you'll find many baroque fugues which violate many of these rules. Check Bach's BWV 899 Fughetta in Dm: the subject is simply "D - E - F - E - D".
As for me, some other good points must be considered regarding fugue subject, such as:
- Do you like your subject? Believe me, it's not that uncomonn to find people composing on themes which they don't like...
- Does it have a good motivic material? Remember that you'll use it later to develop your fugue.
- It's nice when you have a "unique feature" in your fugue! Of course it won't be really inedit and unique, but it will be easily remembered, like a surprise/insistent triplet, or a trill, or even a weird interval! Try to make it interesting/intriguing to the listener ^^
Well, I believe I've done my contributions. I hope it can be useful^^