Deoraiso or Derabonon?
1. Some poskim hold that just like Kiddush, the chiyuv of Havdoloh is deoraiso, based on the posuk, "זכור את יום השבת לקדשו," which commands us to remember and sanctify Shabbos both when it enters and when it leaves (רמב"ם פכ"ט שבת ה"א, סמ"ג מ"ע כ"ט וחינוך). Others hold it is derabonon [and the posuk is an asmachto] (מגיד משנה בשם י"מ, ארחות חיים ועוד). However, Chazal decreed that it should be said with a cup of wine just like Kiddush (מ"ב סי' רצ"ו סק"ו).
2. All poskim agree that after a person is yotzei his chiyuv deoraiso of Havdoloh by saying it in Shmoneh Esrei or by saying "Boruch Hamavdil bein kodesh lechol," Havdoloh with a cup of wine is only derabonon (מ"ב שם). However, some say it is assumed that we have in mind to fulfill our chiyuv deoraiso specifically with Havdoloh on a cup of wine, the way Chazal decreed it should be done [we find a precedent for this concept in the halochos of Kiddush] (שו"ת חת"ס או"ח סי' כ"א).
Havdoloh before Maariv
3. Lechatchiloh, a person should follow the order set forth by Chazal and daven Maariv before saying Havdoloh (מג"א סי' רצ"ג סק"ד, סי' רצ"ט סק"ט). Additionally, based on the Zohar, one should not light a candle before saying the Kedushoh in Uva Letziyon at Maariv (סי' רצ"ג סק"ג).
4. When necessary. If there is a need or someone feels weak, etc., one may say Havdoloh and drink the wine (שו"ע הרב סי' רצ"ג סק"א, פאר ישראל ח"א עמ' ר"ז) or give it to a family member to drink (ארחות רבינו א"ח עמ' קכ"ח) and then daven Maariv. Even when one says Havdoloh before Maariv, he should say Atoh Chonantonu in Maariv (מ"ב סי' רצ"ד סק"א).
