I have previously mentioned that Section 271 is a very long section and much of it is no longer relevant to the usual kinds of situation in which we may find ourselves today when reciting Kiddush in our homes. That is why we now jump from paragraph 2 to paragraph 4. (The omitted paragraph is concerned with what should be done when one is not able to procure both wine and food.) And even paragraph 4 itself contains extraneous matter which we have omitted. We have joined the relevant part of paragraph 4 with its natural continuation in paragraph 7 and omitted all the intervening extraneous matter.
The provisions of these two paragraphs, 4 and 7, are very straightforward and will require very little explanation on my part. As we learned at the very beginning of Section 271, the sages require us to recite Kiddush before our repast as soon as possible after the evening service - immediately upon returning home. From the moment that Shabbat begins (whenever that might be) the duty of reciting Kiddush becomes effective and must take precedence over everything else: in Shabbat 025 we explained that the Torah requires us to 'mention' the sanctity of Shabbat at its very onset and that the sages instituted that this special 'mentioning' should, preferably, take place 'over wine', just before eating our meal.
If the duty of reciting Kiddush becomes effective as soon as Shabbat begins one should proceed to fulfill the Mitzvah as soon as possible and not let other considerations, such as satisfying temporary hunger or thirst, from fulfilling the Mitzvah. If night has already fallen then obviously Shabbat has already started and Kiddush must be recited with as little delay as possible. But it is quite possible that the duty to recite Kiddush will become effective even before nightfall: if one elected to accept the sanctity of Shabbat at some time between Pelag ha-Minĥah and nightfall then the duty of reciting Kiddush will become effective even though it is still broad daylight, and even if the sun is still shining! (See Shabbat 013 for a fuller explanation.)
Paragraph 7 serves to correct a possible misunderstanding of the instructions in Paragraph 4. Rabbinic legislation recognizes that in many situations two possibilities of addressing the law may apply: what is the law ideally? and should the provisions of the law still apply even if one did not observe the law ideally? These two possibilities are known in Hebrew as lekhatĥilah and bedi'avad respectively. A useful translation of the two terms might be "in the first place" and "if it were done". In other words, lekhatĥilah signifies the law as it should be applied when it is carried out in the best possible fashion; bedi'avad signifies what the law should be in less ideal circumstances. In our case, lekhatĥilah one should not taste anything before Kiddush; but bedi'avad - if someone did taste something before reciting Kiddush this does not disqualify them or absolve them from the duty of reciting Kiddush - immediately, or as soon as they realize that they have not recited Kiddush.
Paragraph 8 is a further elaboration of the bedi'avad situation. What would be the situation of someone who, for any reason, did not recite Kiddush at all during Friday night? Perhaps they forgot to do so; perhaps they were prevented from doing so by some emergency. From everything that we have said so far in this shiur it should be clear that duty to recite Kiddush applies throughout Shabbat. Since essentially Kiddush should be recited as the immediate preliminary to a meal, the most logical solution seems to me that such a person should recite the Kiddush appropriate to Friday night before their noontime meal on Shabbat (instead of the much shorter Kiddush which is customary at that time). Rabbi Moshe Isserles, in his note, explains that the whole of the Kiddush (a translation of which I provided in Shabbat 025) should be recited, but that the introductory biblical quotation from the book of Genesis should be omitted, since by that time it would no longer be relevant. It seems to me that in such unusual circumstances no harm would be done if the text of the Friday night Kiddush is preceded by the quotation from the bible [Exodus 31:16-17] which is customarily recited before the Shabbat noon repast.