This is also the reason is taken by most commentaries as a reference back to the matter of conscience.

That you pay taxes is taken by most commentators and translators as an indicative, though the form may be imperative (see Jerusalem Bible “the reason why you must pay taxes” and Phillips “it is right, too, for you to pay taxes”—these seem to be the only translations that interpret this as an imperative).

The authorities is literally “they” in the Greek text but a number of translations make this pronominal reference explicit (see Revised Standard Version, Phillips, Moffatt, New English Bible).

Working for God is literally “servants of God.” Originally the word “servants” was used of government officials, but later it was used in a wider sense, including priests (see Hebrews 8.2) and servants in general (see 15.16; Philippians 2.25). Most translators prefer to maintain the noun phrase here rather than to transform it into a verb phrase as is done in the Good News Translation.

When they fulfill their duties is literally “as they are constantly giving attention to this very thing.” “This very thing” has reference to what precedes (that is, to the general responsibility of rewarding the good and of punishing the evil) and not to what follows (Jerusalem Bible “they serve God by collecting taxes”). It is better to render this phrase in a more general way, as most translations do (see New English Bible “and to these duties they devote their energies”; An American Translation* “devoting themselves to this service”), or to make explicit the reference to what precedes (Phillips “for the good purpose of public order and well-being”). It is advisable not to follow the Jerusalem Bible at this place. In some languages this final clause of verse 6 is translated as “when they do their work” or “when they do what they should do.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .