A grave is a location where a dead body (typically that of a human, although sometimes that of an animal) is buried. Graves are usually located in special areas set aside for the purpose of burial, such as graveyards or cemeteries.
It is narrated that, before he passed away, Imam 'Ali (A) said that when they dug his grave, they would find a white rock shining with light and a shield, and that he should be buried by the shield. When the grave for Imam Ali (A) was dug, they found a white rock shining with light and a shield and on which was engraved: "This is one of the things which Nuh stored for 'Ali ibn Abi Talib."
This is related in Kitab al-Irshad (you can find the English edition online) and Bihar al-Anwar (vol. 42, p. 217).
Obviously, none of us were alive during the time of Nuh (A) to see with our own eyes whether or not this is true. However, it is possible that it happened, and there is no reason why it could not have happened.
However, it would have happened much more than 700 years before the time of Imam 'Ali (A) because Nuh lived a long time ago, although it is not certain exactly when he lived. Islamic narrations place him as being about 9 generations after Adam (A), although it should be kept in mind that people were said to have lived longer then.
Even if the heirs allow, it is not permissible to dig an existing grave to bury another dead body if the previous dead body still exists in the grave. If the previous dead body has completely decayed and became soil, then it will be permissible to bury another dead body in the grave which has no remaining. In such case, if the land of the grave was purchased by the heirs, their permission is must to bury another dead body in that empty grave.
The grave is the beginning stage of Hereafter (Aakhirah). The grave of the believer is a garden from the Paradise, while the grave of the disbeliever is a part from the Hellfire.
Talqeen which is recited for very dead person mentions the main questions which are: Who is your Lord? Who is your prophet? Who are your Imams? etc.
The questions about the faith are the main questions in the grave. Nevertheless, our deeds will have an effect in out grave. Good deeds will have very good reflection in the grave, while bad deeds will have bad reflection as well.
"…Said those who prevailed in the matter, "We will surely take [for ourselves] over them a masjid." Quran 18:21
We can understand from this ayah, about the people of the cave, the permissibility of building masajid at burial sights of respected individuals.
Also, refer to Kamil al Ziyarat for many narrations about the significance of visiting Imam Hussain's (as) shrine.
No one from us can suggest the exact nature of time in the grave, and how dead persons feel the time in their graves, simply because we have not yet experienced it.
What we know for sure according to Quran and authentic Hadeeths that the grave of the believer will be like a garden from Paradise, while the grave of non believer will be a ditch from the hellfire. No doubt, there is morning and evening felt in the grave because we read in Quran that the punishment of Firaon and his supporters will be exposure to fire in the morning and evening النار يُعرًضون عليها غُدوا وعًشيا.(Sura 40, verse 46).
No, it was not shifted. Ruqayyah Bit Al-Husain (AS) passed away in the same place where her grave is now. Yazeed put the captives from Ahlul Bayt (AS) when they reached to Shaam in a ruined ground not too far from his palace in Damascus. Ruqayyah was crying for her father Imam Husain (AS). Yazeed sent to her the head of her father covered in a tray. She got deadly shock when she saw the head of her father and passed away. She was buried in the same place.
May Allah bless you and your family.
Yes, we have a ḥadīth in that vein:
یَا ابا ذر! کُنْ کَأَنَّکَ فِی الدُّنْیَا غَرِیبٌ أَوْ کَعَابِرِ سَبِیلٍ، وَ عُدَّ نَفْسَکَ مِنْ أَصْحَابِ الْقُبُورِ
“Abū Dharr! Be like a stranger in this temporary life or like you are a mere traveler. And consider yourself like one of the people of the graves.”
This phrase is part of a longer ḥadīth found in Makārim al-Akhlāq of al-Ḥasan ibn Faḍl al-Ṭabarsī, the son of the author of the tafsīr entitled Majmaʿ al-Bayān. You can read a translation of the ḥadīth here: https://www.al-islam.org/articles/advice-prophet-s-gave-abu-dharr
The phrase “count yourself like one of the people of the graves” means to live as if death is imminent—or, more directly, act as if you are about to die. In other words, live a pious life because you don’t know when you are going to die and return to your Creator and be held accountable for your misconduct. Never take a day or even a moment for granted and expect that if you sin, you will have a moment after the sin to repent. Act as if, at any moment, you may be lowered into your grave and have no second chance to live a pious life.
One way to encourage this line of thinking is to visit a Muslim graveyard or the graves of deceased family members and contemplate how close death is and how close the afterlife is—how short our lives are. This should encourage us to, at bare minimum, complete our obligations (wājibāt) and refrain from all that is prohibited (muḥarramāt).
Yes, it is allowed, although I am not aware of any traditions recommending it.
However, I think it is better not to use plastic flowers (which you see in some places instead of real flowers) for environmental reasons.
We do not have any Hadeeth or narration or practice of Ulama to place flowers on the grave. It is a habit from non Muslims. It can be waste of money. The deceased will get no benefit from it. It is good to pay the cost of the flowers to poor and needy on behalf of the deceased.
Building on the graves of pious people is mentioned in Quran (Those who overcame said we will build on the (graves of People of Cave) a Masjid.) (Sura al-Kahf, Verse 21.
All Muslims believe that building on the graves of the pious is highly recommended, that it why you see the tomb on the grave of the Prophet Muhammad (SAWA) and all the Infallible Imams and famous personalities of all Muslims sects like the tomb on the grave of Abu Hanifa in Baghdad, tomb on the gave of Shaafi'ee in Cairo, tomb on the grave of Ahmad Ibn Hanbal in Baghdad, tomb on the graves of Abdul Qadir Gilani in Baghdad and hundreds of tombs which belong to all Muslim sects except Wahhabism who objected on building on the graves with out any real evidence.
Bismihi ta'alaYou will be able to find a detailed response to your question in my short series on Life after Death:
With prayers for your success.