Learn Portuguese (Portugal): O engenheiro está na obra enquanto os operários constroem. - The engineer is at the construction site while the workers build.

Enquanto means "while" or "as long as". When it describes two things happening at the same time in reality, like in this sentence, it just uses the normal present tense (the indicative). You only need to worry about the subjunctive with enquanto if you are talking about the uncertain future.

Why use está instead of é for "is" at the construction site?

We use está (from the verb estar) because it describes a temporary physical location. The engineer is physically located at the site right now, but being there is not a permanent characteristic of who he is.

Why is "at the" translated as na here?

Na is the mandatory contraction of the preposition em (in, on, or at) and the feminine word for "the", a. We use the feminine a because obra (construction site) is a feminine noun.

Constroem looks like an unusual spelling for a verb ending. What verb is this?

It is the "they" form of the verb construir (to build). Verbs ending in -uir have some irregular forms in the present tense. For "they" (which applies to os operários), the ending becomes -oem, making it constroem.