Original: Even this simple rule creates issues requiring special cases; for example, d is, specifically, an abbreviation of the verb do; the noun do representing the note of the musical scale is a different word, and must be spelled out.
Simplified: Even this simple rule has exceptions and special cases, which must be thought of. As an example, only the verb to do is replaced by d, the noun do (which stands for a note in music) is a different word, and is written in full.