Salam languagemaniac
5. But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, and seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent, and establish regular prayers and pay Zakat, then open the way for them: for Allah is Oft- forgiving, Most Merciful.
why are establishing prayers and paying zekat obliged to do whereas in one of the verses is said:there is no compulsion in religion?
It is one of the most out-of-context quoted verse of the Quran. I have a feeling that you might have just read it from an anti Islamic website and not directly from the Quran. I may be wrong but I don't think I am because the next verse (verse 6) clears it up. But anyway, I'll give you the context of these verses. The Muslims and the pagans of Makkah had a peace agreement which was unilaterally broken by the pagans of Makkah. They killed some unarmed Muslims. Allah is saying here that this is a declaration of war. When you find these pagans of Makkah, you'll have to kill them in the battlefield. Allah was not telling the Muslims to force Islam on the pagans. He was merely telling them to retaliate against those who broke the peace treaty. Isn't that justified? Now normally the war should continue until the hostile pagans are finished. But Allah has given them an option that if they become Muslims, they won't be harmed. That is what is given in this verse.
009.005 فَإِذَا انْسَلَخَ الأشْهُرُ الْحُرُمُ فَاقْتُلُوا الْمُشْرِكِينَ حَيْثُ وَجَدْتُمُوهُمْ وَخُذُوهُمْ وَاحْصُرُوهُمْ وَاقْعُدُوا لَهُمْ كُلَّ مَرْصَدٍ فَإِنْ تَابُوا وَأَقَامُوا الصَّلاةَ وَآتَوُا الزَّكَاةَ فَخَلُّوا سَبِيلَهُمْ إِنَّ اللَّهَ غَفُورٌ رَحِيمٌ
009.005 But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, and seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent, and establish regular prayers and practise regular charity, then open the way for them: for Allah is Oft-forgiving, Most Merciful.
So what about the pagans who don't want to fight and they don't want to accept Islam. They only want peace. The next verse tells us what to do with them.
009.006 وَإِنْ أَحَدٌ مِنَ الْمُشْرِكِينَ اسْتَجَارَكَ فَأَجِرْهُ حَتَّى يَسْمَعَ كَلامَ اللَّهِ ثُمَّ أَبْلِغْهُ مَأْمَنَهُ ذَلِكَ بِأَنَّهُمْ قَوْمٌ لا يَعْلَمُونَ
009.006 If one amongst the Pagans ask thee for asylum,
grant it to him, so that he may hear the word of Allah; and then escort him to where he can be secure. That is because they are men without knowledge.
Al-Qur'an, 009.005-006 (At-Tawba [Repentance, Dispensation])
Text Copied from DivineIslam's Qur'an Viewer software v2.910
Molana Maududi tells us the explanation of this verse.
That is, "lf during a fight, an enemy makes a request that he should be given an opportunity to understand Islam, the Muslims should give him a surety of protection and allow him to visit them. They should then present Islam before him in order to make him understand it. If, after this, he dces not embrace Islam, they should convey him safely to his place" . Such a person who comes to Dar-ul-Islam under the above mentioned protection is called musta'min in the Islamic Code.
http://www.islamicstudies.info/tafheem.php?sura=9
So as Allah says in the QUran
002.256 لا إِكْرَاهَ فِي الدِّينِ قَدْ تَبَيَّنَ الرُّشْدُ مِنَ الْغَيِّ فَمَنْ يَكْفُرْ بِالطَّاغُوتِ وَيُؤْمِنْ بِاللَّهِ فَقَدِ اسْتَمْسَكَ بِالْعُرْوَةِ الْوُثْقَى لا انْفِصَامَ لَهَا وَاللَّهُ سَمِيعٌ عَلِيمٌ
002.256 Let there be no compulsion in religion: Truth stands out clear from Error: whoever rejects evil and believes in Allah hath grasped the most trustworthy hand-hold, that never breaks. And Allah heareth and knoweth all things.
Al-Qur'an, 002.256 (Al-Baqara [The Cow])
Text Copied from DivineIslam's Qur'an Viewer software v2.910
I urge you to read the Quran in context.
Peace