This Beautiful and lesson-giving Shabad appears in Raag-Tilang, in Siri Guru Granth Sahib jee. This is one of the 4 Shabads of Siri Guru Nanak Dev jee that are collectively called Babar Baani because these Shabads relate to Babar's attack on Hindustan.
One Shabad and two Ashtpadis appear in Raag Asa and one Shabad appears in Raag Tilang. The Shabad that appears in Raag Tilang is the first Shabad and is a Peshangoee (telling the future). When Guru Sahib went to Saidpur (Aimnabad), the town of Bhai Lalo and witnessed that the people of this town were totally indulged in the pleasures of the world and did not remember Vaheguru at anytime, Guru Sahib foretold of the destruction that this town was inviting as a result of its sinful behavior. If we look at the whole Shabad we notice that the first half of the Shabad is addressed to Bhai Laalo. Daas, by the Kirpa of Guru Sahib intends to write a detailed Vichaar on Babar Baani but for now let's discuss the intelligent questions you have raised in your post.
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ਸਾਹਿਬ ਕੇ ਗੁਣ ਨਾਨਕੁ ਗਾਵੈ ਮਾਸ ਪੁਰੀ ਵਿਚਿ ਆਖੁ ਮਸੋਲਾ ॥
Nanak sings the Glorious Praises of the Lord and Master in the city of corpses, and voices this account.
What does Guru Sahib mean by "city of corpses"?
The translation is wrong. Maas-puri has been wrongly translated as "city of corpses". Maaspuri means the city of meat i.e. this body.
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ਕਾਇਆ ਕਪੜੁ ਟੁਕੁ ਟੁਕੁ ਹੋਸੀ ਹਿਦੁਸਤਾਨੁ ਸਮਾਲਸੀ ਬੋਲਾ ॥
The body-fabric will be torn apart into shreds, and then India will remember these words.
What Guru Sahib mean by "torn apart into shreds", and what words india will remember?
When Guru Sahib saw that both Sharam and Dharam had vanished from the land, He predicted bodies will be cut into pieces i.e. many will die when they will get attacked. Guru Sahib is saying that Hindustan will remember His prediction that destruction will be coming to this land soon, as a result of their sinful activities.
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ਆਵਨਿ ਅਠਤਰੈ ਜਾਨਿ ਸਤਾਨਵੈ ਹੋਰੁ ਭੀ ਉਠਸੀ ਮਰਦ ਕਾ ਚੇਲਾ ॥
Coming in seventy-eight (1521 A.D.), they will depart in ninety-seven (1540 A.D.), and then another disciple of man will rise up.
What does Guru Sahib actually mean, or what is He actually telling us? What happenend in 1521 A.D. and 1540 A.D.? And who rose up?
Guru Sahib is writing that in Sammat 1578, they aggressors will come to India and bring destruction but they will be driven away in Sammat 1597 when Mard-ka-Chela Sher Shah Suri will rise. History is witness to this accurate prediction of Guru Sahib and this is exactly how it happened. Babar came in 1578 Sammat and the Mughals were driven out by the Pathan leader Sher Shah Suri in 1597.
If Guru Sahib permits, a more detailed Vichaar on Babar Baani will be done.
By the way, other Shabads of Babar Baani are as follows:
ਖੁਰਾਸਾਨ ਖਸਮਾਨਾ ਕੀਆ ਹਿੰਦੁਸਤਾਨੁ ਡਰਾਇਆ ॥ (360)
ਕਹਾ ਸੁ ਖੇਲ ਤਬੇਲਾ ਘੋੜੇ ਕਹਾ ਭੇਰੀ ਸਹਨਾਈ ॥ (417)
ਆਦੇਸੁ ਬਾਬਾ ਆਦੇਸੁ ॥ (417)Daas,
Kulbir Singh