Romans 12 18: "if it be possible as much as lieth in you live peaceably with all men"
This is the only commandment with a caviott like this.
Romans 13 8: "owe no man anything but to love one another for he that loveth another hath fulfilled the law"
This verse clearly teaches owing debt is a sin. If you have a mortgage or other debts, pay them off as soon as possible!
Romans 14 5 and 6: "one man esteemeth one day above another another esteemeth every day let every man be fully persuaded in his own mind he that regardeth the day regardeth unto the Lord and he that regardeth not the day to the Lord he does not regard he that eateth eateth to the Lord for he giveth God thanks and he that eateth not to the Lord he eateth not and giveth God thanks"
This passage has been used to try to prove the sabbath has been abolished. However, what Paul is saying here is, "One man only attends church on the sabbath, while another man goes to sabbath service, Bible study, prayer meeting, and everything else that's going on. Let both be convinced that what they're doing is right in the eyes of God. He that goes to every activity is doing what the Lord told him to do, and he that only goes one day a week is doing what the Lord told him to do. In the same way, one man is OK with eating meat and thanks God by saying grace before he eats it, and another man won't eat any meat because he doesn't want to eat something that might have been sacrificed to an idol, he gives thanks to God for whatever food he eats instead. The point is, in both examples, both men are brothers doing what the Lord has told them to do."
Romans 14 13: "let us not therefore judge one another any more but judge this rather that no man put a stumblingblock or an occasion to fall in brother's way"
When it comes to weaker Christians, meaning Christians that think something is wrong even though the Bible doesn't condemn it, we should not do things around them which will make them think we are false brethren. For example, the Bible does not condemn drinking alcohol, only getting drunk. However, around Christians who we know think that consuming any amount of alcohol is wrong, we should not drink because this will cause them to think we are not sincere in our faith.
Of course, it is possible for a Christian to be strong in most areas but be weak in one or a few areas.
Romans 14 23: "and he that doubteth is damned if he eat because not of faith for whatsoever not of faith is sin"
Whatever does not proceed from faith is a sin. This passage reminds Christians that they have to be very careful about the reasons why they're doing something. We shouldn't go along with things, not even things that aren't in themselves sinful, just because we've been pressured by someone else.
Romans 15 4: "for whatsoever things were written aforetime were written for our learning that we through patience and comfort of the scriptures might have hope"
The Old Testament is there for us to learn things. The Old and New Testaments don't contradict each other. Thus, things such as the imprecatory prayers in the Psalms have application today, just as they did when they were written.
Romans 15 22: "for which cause also I have been much hindered in coming to you"
In the beginning of this letter, Paul talks about how he has been wanting to visit the church at Rome, but was unable due to circumstances (Romans 1 10-13.) He then goes on to give all this theology which underlies Christianity, from how man didn't like to retain God in their knowledge and were given up to a reprobate mind, to Adam versus Christ, to the law of the Spirit, to what obeying that law and living as brothers in Christ looks like, to how he's been given the task of preaching to the Gentiles, and now he finally says, "So that's the reason I haven't been able to come see you." Praise the Lord!