וַיִּהְי֥וּ בְנֵֽי־יִשְׂרָאֵ֖ל בַּמִּדְבָּ֑ר וַֽיִּמְצְא֗וּ אִ֛ישׁ מְקֹשֵׁ֥שׁ עֵצִ֖ים בְּי֥וֹם הַשַּׁבָּֽת׃
Once, when the Israelites were in the wilderness, a fellow was found gathering wood on the sabbath day.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of אִישׁ, by employing a situation-oriented construal as outlined in “Notes on Gender in Translation,” pp. 11–16.)
Prototypically, אִישׁ is used when sketching a situation schematically. This is such an instance. It introduces a new participant in the situation, profiling him simply as an essential element in that situation. (The referential gender of אִישׁ is manly because the reference is to a specific individual.)
The previous clause sets up the situation, pointing to the discourse-active group בְּנֵי־יִשְׂרָאֵל. That context implies that the newly introduced participant in וַיִּמְצְאוּ אִישׁ is now being individuated as a member of that group. In the process, the verb’s unstated plural subject is impersonal (akin to they in “Do they say it’s going to rain today?”). It is not to be construed as applying to all of “the Israelites” (mentioned in the previous clause), as the next verse makes clear.
As for translation, the NJPS rendering “they came upon a man” raises the question as to how we are supposed to know that the figure in question is an Israelite. The implication is that he is not—that “the Israelites” happened upon someone else, rather than one of their own. This is significantly misleading, because the episode turns on the implicit fact that laws about keeping the Sabbath apply only to Israelites. The revised rendering avoids the misleading implication.