Erm dunno...tough question. I find most of the time that after the first line everythin comes easier coz you get a feel for how you want it to sound and flow. You get me? I think the opener is always the hardest. I find that a chorus is always hard aswell unless you got it written down somewhere.
But maybe writin to a beat is easier....sometimes. It depends on what mood your in and what type of beat it is...upbeat,slowbeat...whatever.
But if you got writers block then nothin works....damn I hate that shit

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HARRY R.I.P
1940 - 2002

In Respect