What is the ghayr muakkad sunnah? What is the mandub in islam? What are the acts of ghayr muakkad sunnah?
Sunnahs that the Prophet (saw) performed at most times but sometimes abandoned are as follows:
1) As mentioned above, two cycles of prayer are performed in addition to the two-cycle mu’akkad sunnah performed after the farḍ of the noon prayer.
2) A four-cycle prayer is performed with a single greeting before the late afternoon prayer. The Messenger of Allah (saw) said about this prayer, “May Allah have mercy on a person who prays four cycles before the late afternoon prayer.”
3) Four-cycle prayer performed before the night prayer. The evidence for this is the following hadith reported by Aisha (r. anha): “The Messenger of Allah (saw) used to perform four cycles before the night prayer, then pray four more cycles after the night prayer, and then go to bed.”
These are supererogatory prayers that are attached to the obligatory prayers. The independent nāfilah prayers will be explained below.
 Al-Tirmidhī, Ṣalāh, 301.  See al-Zaylaī, Naṣb, II, 145 ff.; al-Shawkanī, ibid, III, 18.
Source: Basic Islamic Principles (ilmiḥal) According to the Four Sunni Schools With Evidence From The Sources of Islamic Law, Prof. Hamdi Döndüren, Erkam Publications