The focus is now on Bacchides, so translators may begin a new paragraph here (so Revised Standard Version, Good News Bible).

When Bacchides heard of this: Bacchides heard that Jonathan, Simon, and their men were going down to the river; he knew exactly where he could find them. This clause may be rendered “When Bacchides heard where they were going.”

He came with a large force on the sabbath day to the banks of the Jordan: In verse 34 we were told that Bacchides had crossed over the Jordan to the east side, presumably because he had reason to think the Jewish fighters were going there (see the comments there). The sabbath day is mentioned because in 1Macc 2.41 it says the Jews had determined that they would not hesitate to defend themselves on the Sabbath if they were attacked. The implication may be that they themselves would not attack on the Sabbath. If Bacchides knew this, he might have felt free to move his troops on the Sabbath without fear of a surprise attack. It may also explain why the Maccabees did not cross the Jordan into Judea at that point; they did not want to violate the Sabbath by unnecessary travel.

An alternative model for verses 42 and 43 is:

• 42~After Jonathan and Simon had taken full revenge for the death of their brother, they and their men started back [to Judea]. They got as far as the marshes [or, swamps] along the Jordan River, 43~when they met [or, found themselves facing] Bacchides with a large army. He had learned where they were, and had come down to the river [to meet them]. This happened on a Sabbath.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.