Modern music is defined by a general abandonment of the rules that were laid down in earlier times. Where Romantic music had irregular phrase lengths, modern music often goes one step further and uses irregular time signatures, or changes of time signature every few bars, or a lack of time signature. Tonal harmony was also dismantled by some composers, who wrote music that does not necessarily fit into the major/minor diatonic key system; some music is based on other types of scales, like the whole tone or pentatonic scale, serialism (12-tone music) or some pieces are just simply atonal – not based on any type of scale or key. Sometimes the emphasis of the music is simply rhythmic, rather than melodic. Modern music is often unpredictable and highly inventive.
Because of this, modern era melodies and harmonies are often built on dissonant intervals like augmented or diminished intervals, or 2nds and 7ths. You may also find clashing notes or clustered chords that seem impossible to name.
Modern composers are often quite inventive when it comes to finding ways to get new sounds out of old instruments, so you might find techniques like flutter tonguing on the flute, or extremely high-pitched harmonics on string instruments. You can also expect to find very detailed performance directions, often written in the composer’s own language, rather than using only the standard Italian terms. Instruments might also be used in unconventional combinations.