For indicates that Paul is about to sharpen the general criticism of verse 17. Barrett translates “Here is a particular example.”

In the first place, in Greek as in English, suggests that Paul is beginning a list, as for example in 12.28. There is no clear indication that Paul ever continues the list, though he passes to other criticisms related to worship at Corinth in chapters 12 and 14.

When you assemble as a church comes at the beginning of the verse in Greek and is rather emphatic. The same phrase is repeated in verse 20, “when you meet together.” The similarity in thought and language between verses 18a and 20a suggests that the phrase as a church (Good News Bible‘s “in your meetings”) and “together” (20a) are similar in meaning. Commentators discuss whether or not these phrases refer to an actual place. It seems unlikely, however, that Paul is referring to a special place where Christians met in Corinth for worship. He is, nevertheless, referring to physical meetings of the Corinthian Christians, not merely to a unity of spirit. In the New Testament, as in the Septuagint, the Greek word for church often refers to God’s people gathered for worship. This is the likely meaning here. See also the comments on verse 20.

I hear that may also be translated as “People have told me.”

The word factions (Good News Bible “divisions”) implies that the Corinthian Christians were dividing themselves into groups in their meetings and opposing or arguing with each other. The first part of the verse may be restructured in the following way: “The first thing I want to mention is that people have told me that when you meet together you split up into groups and oppose each other.”

I partly believe it may be translated as “I am sure that there is some truth in what they say.”

This verse can be restructured in the following way:
• “In the first place, people have told me that when you meet together you split up into opposing groups. I believe that there is some truth in this.”

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .