Χαίρετε!
In Dr. Mastronarde’s Unit 27, Part I, Exercise 1, I am supposed to write,
"At the command of the general [use absolute participial construction], everyone marched along the river, keeping on guard.
I wrote, “κελεύοντος τοῦ στρατηγοῦ, πάντες τὸν ποταμὸν ἤλαυνον φυλάττειν.”
The answer book wrote, “κελεύσοντος τοῦ στρατηγοῦ πάντες παρὰ τὸν ποταμὸν ἤλαυνον φυλάττομενοι.”
I used an accusative of space, but the answer book threw in παρὰ. Is that necessary?
I used the infinitive of φυλάττω while the answer book used the participle. How do I know which to use?
There have been several examples in M. of παρὰ τὸν ποταμὸν eg on p. 72. with the sense of “alongside the river”.
The example M. gives for accusative of space is ἄγει τὴν στρατιὰν στενὰς ὁδούς. He leads the army along (over) narrow roads. p. 142.
I think you can see that the marching could not take place “over the river” nor along (the length of) , unless there was some miraculous intervention or the river had run dry as often happens in Greek rivers in the Summer. παρὰ τὸν ποταμὸν emphasises where the army was, ie on the river bank. I would hesitate to say what you wrote was wrong but M. is more specific and clearer.
I agree with what Seneca wrote, but are you sure the key has κελεύσοντος and not κελεύσαντος? An aorist participle seems more appropriate than your present or, even less, a future.