He frowned and began an explanation that Serezha had heard many times already and could never remember because he understood it too clearly, just as he could not remember that the word suddenly was an attribute of the manner of action.
Arrugó el entrecejo y empezó a decir lo que Sergio oyera ya cien veces y no podía recordar por comprenderlo demasiado bien, al estilo de la frase «de repente», que era un modo adverbial.