Would you please describe briefly the situations when Sajda e Sahw is mandatory?

الجواب وباللہ التوفیق

Following are the reasons for the Sajdah e Sahw to be Wajib in Salat. When any one of these reasons will be found then the Sajdah e Sahw will become wajib:

1. Performing any Fard or Wajib A’mal earlier than its actual place, e.g., performed Ruku’ before the recitation of Qura’an or recited Surah before the recitation of Surah Fatihah.
2. Performing any Fard or Wajib A’mal later than its actual place, e.g., forgot one Sajdah in the first Raka’t and when recalled performed three Sajdahs in the second Raka’t, or recited Surah Fatihah after the Surat.
3. Performing a Fard or Wajib repeatedly, e.g., performed Ruku’ twice, or performed three Sajdahs in one Raka’t.
4. Changing the quality of a Wajib, e.g., the Imam did Qirat in low voice when it was to be done in loud voice or did Qirat in loud voice when it was to be done in low voice.
5. Abandoning any Wajib, e.g., did not recite Tashahud, or left Surah Fatihah.

(حلبی کبیر ۴۵۵، ۴۵۶، شامی بیروت ۲؍۴۷۴ – ۴۷۵ ، شامی زکریا ۲؍ ۵۴۳ – ۵۴۴) (بحوالہ کتاب المسائل)

Other situation could also occur necessitating Sajdah e Sahw, inquire the ruling about them when needed.

واللہ اعلم بالصواب