If someone didn’t hear words (or perhaps even an entire verse) from Parshas Zachor or Parshas Parah, are they yotzee b’dieved?
There is a dispute among opinions concerning this question. The principle opinion appears to be that one is still yotzei, provided the main theme was heard.
Of course one should be careful to hear every word, and the custom is clearly to be careful for the precise reading of every word, even those that are not essential for the understanding of the passage.
According to the opinions that reading Zachor (specifically as a public Torah reading; this is based on the Rosh) is a Torah obligation, some write that one has to hear every single word of the passage.
For instance, some authorities discuss whether a person also has to understand every word of the passage (see, for instance, Moadim Uzmanim 2:165), clearly implying that one has to hear every word. This is ruled explicitly in Mikra’ei Kodesh (Purim 8), who writes that Zachor is no worse than Megillah, and that one must therefore hear every word. See also Halichos Yisrael (Vol. 1, p. 218), citing from Rav Elyashiv zt”l.
However, Halichos Shlomo (Moadei Ha-Shanah 322:2) cites from Rav Shlomo Zalman Auerbach zt”l that it is sufficient to hear the main theme, and that this emerges from the ruling of the Magen Avraham whereby one can also fulfill the mitzvah by hearing the reading of Amalek in Parashas Beshalach. The same ruling is given by Moadei Yeshurun (1:47:7) citing from Rav Moshe Feinstein zt”l.