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Siman Nun Vav Seif Aleph in the Shulchan Aruch is entitled: "The
Halachot Regarding Answering Kaddish".
Chapter 56 Section 1: "The Laws of Responding to the Kaddish"
Writes the Mechaber: "One must have proper Kavanah (intentions) during the answering of the Kaddish."
The Mishna Berurah goes into a lengthy discussion in response to this Mechaber: Why do we need such strong kavanah? Chazal teach that one who says "Yehe Shme Rabbah Mevorach with all his might has any heavenly decrees against him nullified." Various Reshonim explain the statement "all of his might" (or "with all of his strength"): that one should use all of his thoughts, soul, and spirit in his response the chazzan, not just a mere utterance of the lips. Additionally, one should follow along with the chazzan in order to answer Amen according to the precise passage read.
(In Seif Samech (section 60) we will discuss the laws of Kavanah (intentions) in further detail, in particular, the question of whether a mitzvah requires kavanah or not. One understanding brought there is that any mitzvah that does not involve a deed requires kavanah. That is, any mitzvah that requires thought or words alone requires kavanah. This is what we are saying here in regards to the kaddish. The response to the kaddish consists only of words, and so one must have proper kavanah when reciting them.)
The Mishna Berurah goes on: "One must be extremely careful not to speak during the Kaddish, or during Kedusha as is brought is Masechet Derech Eretz." The Mishna Berurah then brings several aggadic passages to drive home his point in this regard.
"Rebe Chama found Eliyahu HaNavi in the field bringing along thousands of camels and wagons that were carrying a tremendous amount of anger."
Our sages explain: If we imagine Olam Haba (the world to come) as a place where our deeds are brought in a physical form, then in this case they are manifest as carts and wagons. These wagons represent the physical form of the speech that came in front of the heavenly court to represent people's actions of talking during the Kaddish and Kedusha. This speech during kaddish and kedushah resulted in these wagons full of condensed anger - the anger of the Heavenly Court wrought upon those who speak inappropriately at these points in davening.
Another agadah, from the Sefer Chasidim:
There was as story of a chasid (pious person) who passed away and appeared to his friend in a dream. The friend noticed that his face was green and he looked very sick. So he asked the deceased man, why do you look so ill? He responded "I had talked during the Friday night's chazzan recitation of vayichulu, and during the bracha of Magen Avot and during Kaddish). As a result of his chatter during the tefilah, he appeared sickly in olam haba.
Another agadah from the Mateh Moshe:
A deceased rabbi appeared before his students in dream. His students noticed that he had a stain in the middle of his forehead. They asked him: "Why do have such a stain?" The Rabbi responded: "Because I was not careful to refrain from speech while the chazzan said the kaddish."
How strong is the importance of not speaking, and having proper kavanah (intention) during kaddish?
Says the Mishna Berurah: One should even refrain from the study of torah while the chazzan says the kaddish. The Mishna Berurah explains in the name of the Pri Chadash (Rav Chizkiya ben David DiSilo, 1659-1698, Livorno, Italy): The study of Torah is the greatest of all the commandments, and is incumbent upon us at all times of the day. From here we see the importance of not speaking during the kaddish, and the level of concentration we should aspire to. This obligation of concentration during the kaddish even overrides the commandment to learn Torah at that particular time.
Finally, the Mishna Berurah brings in the name of the Pri Megadim (Rabbi of Berdichev, 1740-1810): There is no difference between the kavanah (intentions) one should have when saying the Kaddish for Tanach, Haggadah or Mishanot.
We have six 6 different types of Kaddish that are said at various times.
IY"H, we will learn more about Kaddish in the coming shiurim.