What does the Qur'an forbid?

Answer 1

From what I observed, the Qu'ran forebids eating pork, drinking alcohol and worshipping other dieties. Those rumors you hear about the Qu'ran forebiding circumcision, are simply due to groups of Muslims making these rules. However, none of it is really recorded in Islamic text. Same with this thing about being in a swimming pool with non-Muslim guys. Islamic scripture has no regulations on that, but simply Muslims, man themselves twist these rules and make it seem like it is recorded in Islamic scripture.
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Quran forbids:

• worshiping partners with God and not believing in God as the one and only one God with no partner, no son, no father, no companion, no associate, and no equivalence.
• adulteryor practicing sexoutsidelicit marriage
• killing innocent people
• robbery and theft
• eating pork
• drinking alcohols
• not helping the poor and needy people
• not respecting parents and treating them rudly
• having bad morals
• not keeping the promise
• laying
• wearinginappropriateclothingthat figures sexy parts anddescribesthe woman body and shows her hair.
• .....

as an example, Quran says (meaning English translation) (Quran, Al-Isra, Chapter 17, verses 22- 39):

"Do not make [as equal] with Allah another deity and [thereby] become censured and forsaken.And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [so much as], "uff," and do not repel them but speak to them a noble word.And lower to them the wing of humility out of mercy and say, "My Lord, have mercy upon them as they brought me up [when I was] small."

(24)

Your Lord is most knowing of what is within yourselves. If you should be righteous [in intention] - then indeed He is ever, to the often returning [to Him], Forgiving.

(25)

And give the relative his right, and [also] the poor and the traveler, and do not spend wastefully.

(26)

Indeed, the wasteful are brothers of the devils, and ever has Satan been to his Lord ungrateful.

(27)

And if you [must] turn away from the needy awaiting mercy from your Lord which you expect, then speak to them a gentle word.

(28)

And do not make your hand [as] chained to your neck or extend it completely and [thereby] become blamed and insolvent.

(29)

Indeed, your Lord extends provision for whom He wills and restricts [it]. Indeed He is ever, concerning His servants, Acquainted and Seeing.

(30)

And do not kill your children for fear of poverty. We provide for them and for you. Indeed, their killing is ever a great sin.

(31)

And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way.

(32)

And do not kill the soul which Allah has forbidden, except by right. And whoever is killed unjustly - We have given his heir authority, but let him not exceed limits in [the matter of] taking life. Indeed, he has been supported [by the law].

(33)

And do not approach the property of an orphan, except in the way that is best, until he reaches maturity. And fulfill [every] commitment. Indeed, the commitment is ever [that about which one will be] questioned.

(34)

And give full measure when you measure, and weigh with an even balance. That is the best [way] and best in result.

(35)

And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight and the heart - about all those [one] will be questioned.

(36)

And do not walk upon the earth exultantly. Indeed, you will never tear the earth [apart], and you will never reach the mountains in height.

(37)

All that - its evil is ever, in the sight of your Lord, detested.

(38)

That is from what your Lord has revealed to you, [O Muhammad], of wisdom. And, [O mankind], do not make [as equal] with Allah another deity, lest you be thrown into Hell, blamed and banished.

(39)"

refer to questions below for more details.

Answer 2

wine, gambling, sin, polytheism (including trinity),...