Sujūd (سُجود, or sajdah (سجدة), is an Arabic word meaning prostration to God (الله Allah) in the direction of the Ka'ba at Mecca which is usually done during the daily prayers (salat). While in sujud, a Muslim is to praise and glorify Allah. The position involves having the forehead, nose, both hands, knees and all toes touching the ground together.
The Prayer is valid.
If you remembered that you missed on Sajda by mistake, you should perform it immediately after the Prayer then perform 2 Sajda of Sahw (forgetting). The way of performing the two Sajda of Sahw is just prostrating and saying during Sajda (BISMILLAH WABILLAH ALLAHUMMA SALLI 'ALA MUHAMMAD WA AALI MUHAMMAD) then sitting and going again to Sajda and saying the same, then sitting and reciting Tashahhud and Salam.
It is permissible to prostrate on a clay on which something is written, though it is much better to prostrate on plain clay ( Torbah) or Mohr or Sajdagah.
No Sajda but only to Allah (SWT).
The Prophet Muhammad (SAWA) and The Infallible Imams refused and forbade Sajda to them and said that Sajda is only to Allah (SWT).
We have many authentic Hadeeths on this fact.
The Sajda of the angles to Adam (AS) was in fact to Allah (SWT) who ordered the angles to do it.
Sajda in the shrines of Ahlu Bayt (AS) is in fact Sajda of Shokr (Thanking Prostration) , to thank Allah (SWT) who granted us he great bounty to reach and visit the shrines of Ahlul Bayt (AS).
Sajda (Prostration) on carpet is Not permissible unless you are under dangerous situations where Taqiyya becomes permissible for you to avoid possible danger or harm.
If you remember the verse in your mind, it is not called reciting but just thinking or remembering. Sajda is obligatory on reciting or listening to any of the known four verses of Sajda.
There is no difference in the obligatory acts of Sajda between man and woman but it is recommended ( Mustahab) for man to raise his arms above the floor putting his hands on the floor while Sajda. For woman, it is recommended for her to put her arms with her hands on the floor while performing Sajda.
It is recommended to recite between the two Sajdas in Prayers: Astaghfirullah Rabbi Wa Atoobo Elaf. أستغفر الله ربي وأتوب إليه.
The Qur'an mentions two types of prostration: (a) prostration as a form of worship and obedience (whether it be literal, as in during prayer; or figurative), and (b) prostration for reasons other than worship (for instance, in some cultures, it was/is a gesture of respect).
Qur'an 2:34, 17:16, and 20:116 say that Allah ordered Shaytan (Iblis) to prostrate to Adam. It literally says "to Adam", and I see no reason why it shouldn't be taken at face value rather than looking for a sideways interpretation. Obviously, Allah is not commanding Shaytan to worship Adam but rather it is a gesture of humility and an acknowledgment of the potential of the human being to ascend higher than the angels. It may also have been a test for Shaytan to prove his inner nature because, up until then, he had been seen as an excellent worshipper.
(Maybe it was also a gesture of service, in that some of the angels, who were also ordered to prostrate to Adam, are assigned to serve people - for instance, in recording the deeds of people. Some people also hold it was out of reverence for the position of the Prophet Muhammad and his Ahl al-Bayt who would be born from Adam.)
Anyway, all of this appears to have occurred outside the earthly realm. Some also say that "Adam" here refers to humanity as an archetype as a whole, as existing outside this world, and not the living, breathing human being. So, while we picture this as Shaytan getting onto his hands and knees and performing what we consider to be sajda to Adam as a human being, it may have been somewhat different. So for this reason also, it isn't really worthwhile to differentiate between "sajda to" or "sajda in front of".
The Qur'an does not specifically say that sajda is only for Allah. Instead, it says that everything in the heavens and earth already does sadja to Allah, willingly or unwillingly (13:52, etc.). This can be taken to mean literal sajda as well as metaphorical. That is, all created beings are compelled to follow the divine decree and no created being can decide it is outside of Allah's control. Most creations willingly worship Allah, although human beings have the free will to worship Allah or other things (and we have the free will to perform sajda to Allah or not perform sajda to Allah). For this reason, the Qur'an tells people not to do sadja to the sun and the moon, and instead tells people to do sajda to Allah who is the creator of the sun and the moon.
However, according to Islamic law, it is not permissible for human beings to do sadja to other Allah. This is derived from Qur'an and hadith.
There is no limit of number of times we can repeat he Thikr or Rokoo' or Sojood. It is recommended to repeat it 3 or 5 or 7 or more in odd times.
According to Shia jurisprudence, Sajda should be performed on earth, and on those things which are not edible nor worn, and on things which grow from earth (e.g. wood and leaves of trees). The Holy Prophet (pbuh) also performed Sajda on soil and other things which are considered to be a part of earth such as sand, clay, stone, plants (wood) etc. not on carpet, rug, leather and plastic materials. The Turbah of Imam Hussein (a.s.) on which Sajda is valid and which has superiority over other soils and things, is one of the manifestations of earth (or soil). Hence, Sajda is allowed on it.
If there is a layer that has formed on the turbah which is a barrier that stops the forehead from reaching the clay, then it must be cleaned or scraped away first
Thank you for your question. As long as the darkness is not due to any barrier between the turbah and the forehead you can prostrate on it. Usually, the darkness is due to water or swear and so the darkness is just discolouration of the clay rather than a barrier.
May you always be successful