Albert Barnes Commentary 1 Thessalonians 3:4

"For verily, when we were with you, we told you beforehand that we are to suffer affliction; even as it came to pass, and ye know." — 1 Thessalonians 3:4 (ASV)

For verily, when we were with you, we told you before, etc. It is not mentioned in the account in Acts 17 that Paul thus predicted that particular trials would come upon them, but there is no unlikelihood in what is said here. He was with them long enough to discuss a great variety of topics with them, and nothing can be more probable than that, in their circumstances, the subjects of persecution and affliction would be prominent topics of discussion. There was every reason to anticipate that they would meet with opposition on account of their religion, and nothing was more natural than that Paul should endeavor to prepare their minds for it beforehand.

That we should suffer tribulation. This refers to us who preached to you, and perhaps it also includes those to whom they preached.

Even as it came to pass, and ye know, which occurred when Paul, Silas, and Timothy were driven away, and when the church was so much agitated by the opposition of the Jews, Acts 17:5–8.