How is night prayer offered?
Night prayer is offering voluntary prayer after Maghrib and before Fajr (Dawn). As for Tahajjud, it is offering voluntary prayer at night after waking up voluntarily, and for Allah`s sake as He Says (What means): "And pray in the small watches of the morning: (it would be) an additional prayer (or spiritual profit) for thee: soon will thy Lord raise thee to a Station of Praise and Glory!" [Al-Isra`/79]. Offering Tahajjud is better than offering voluntary prayer before going to bed.
What should one who doubts the validity of his ablution and performs it frequently do?
Whoever doubts the validity of his ablution after finishing it, his ablution is valid. This is because the default is the validity of the ablution, and because doubt after finishing an act of worship doesn`t affect its validity. And Allah the Almighty knows best.
Who is the one deemed "able" (al-qādir), for whom the uḍḥiyah (sacrificial offering) is sunnah?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
The one who is deemed "able" (al-qādir) [to perform the uḍḥiyah, i.e., the sacrifice] is someone who possesses wealth sufficient to acquire the sacrificial animal, over and above what is needed to cover his own living expenses and the living expenses of his wife and children — or of anyone else whose financial support (nafaqah) is legally obligatory upon him — for the day of ‘Eid and the three days of Tashrīq [that follow it], since this is the designated time period for the sacrifice. And Allah, the Exalted, knows best.
What is the meaning of the Udhiyah?
Praise be to Allah, and peace and blessings be upon our Master, the Messenger of Allah.
The Udhiyah (Sacrificial Offering) refers to the livestock (An'am) that is slaughtered as an act of drawing closer to Allah the Almighty. This takes place on the day of Eid al-Adha (the 10th of Dhu al-Hijjah) and during the three days of Tashreeq that follow the day of Eid. And Allah the Exalted knows best.