This passage occurs in the RCL "Pentecost"/"Ordinary"/"Proper" Season, Year A, most recently September 2014.
Summary: A classic tale of forgiveness. A man owes an absurd amount (Roughly 10.8 billion by my calculations). After being forgiven he arrests his friends who him a couple of hundred dollars. Take your pick: Are we debtors, beggars or slaves? All popular American terms. At least the first one...
δανειον ("debt"; 18.27) The use of this word for "debt" here is unique in the NT. The word normally carries with it a suggestion of interest, even usury with this debt. Most simply it means a loan. God is calling the loan and then forgives it. What has God loaned you!?
ει τι ("whatever" 18.28) The exact construction of the phrase "Pay what you owe me" is rather interesting. It actually includes an "ει τι" phrase. This phrase is normally translated "if anything," as if to say, the man was not even really sure what the debt was, if in fact, it was anything.
παρακαλεω ("encourage"/"plead"; 18.29, 32) This is a powerful theological word used twice in this section. Also used in the present tense. Here the image is one of constant begging (used in present tense). (The word for Holy Spirit is derived from this word: "paraclete")
συνδουλος ("fellow-slave" 18:28, 29, 31, and 33) The Greek can put "fellow" and "slave" together in one word. Powerful word. Fellow slave.
Grammar review: Future vs. Subjunctive: Sins aren't subjunctive in this case!
The Greek language is obsessed with the future. There are multiple ways to show the future implications of a given action. Worth noting is that there is no future subjunctive. Either something will happen in the future or it might happen starting from this moment forward in an unknown time. But you cannot do "might happen in the future"; that simply means might happen. Today, when Peter is asking Jesus about forgiving others, he does not put the verb αφησω (forgive) in the subjunctive. The whole sentence is in the future. In short, Peter expects sin and forgiveness. The sentence literally reads: "How often will my brother against me and will I forgive him? Until seven times?"