For introduces the description of how the Macedonians gave so generously.

They gave: these words are not actually in the Greek text of verse 3; they are introduced from verse 5. But in most languages it will be essential to have such a verb at this point.

According to their means: literally “according to power.” Good News Translation correctly translates the sense as much “as they could.” This expression stands in parallel with beyond their means, which follows. Together they are translated by Contemporary English Version “as much as they could afford and even more.” In some languages the second expression may be translated “they went beyond the limit….”

As I can testify means that Paul serves as a witness; that is, he has personal knowledge of the situation and can therefore speak reliably regarding the Macedonian Christians. Note that Good News Translation shifts this forward to the beginning of the verse, translating “I can assure you….” This may be a good model to follow in other languages.

Of their own free will: these words may be considered a part of the sentence which precedes, that is, the Macedonians gave of their own free will (so Revised Standard Version, New Revised Standard Version, New Jerusalem Bible, Biblia Dios Habla Hoy), or to the sentence which follows in verse 4, that is, of their own free will they begged to take part in the collection (so Good News Translation, New International Version, Revised English Bible, New Jerusalem Bible, Translator’s New Testament, Bible en français courant). The latter interpretation follows the punctuation of the UBS Greek New Testament and is probably more likely.

Quoted with permission from Omanson, Roger L. and Ellingworth, Paul. A Handbook on Paul’s Second Letter to the Corinthians. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .