I'll begin by uttering a rare criticism: this dative of purpose lesson in my opinion has not been explained very well here. That's to say, every time there is a dative like this I need to see in brackets (for), and D'Ooge does include that
Hence I don't understand this concept properly, probably the only thing in which I don't see any logic in the whole course here.
I. 1. Rogavit cur illae copiae relictae essent. Responderunt illas copias esse praesidio castris.
-They replied that those troops were a guard to the camp.
Here I can see in a way why a dative might be used, as the troops are there for a purpose, but can't "esse praesidium castrorum" just be used?
2. Caesar misit exploratores ad locum deligendum castris.
-Caesar sent scouts to select a place for camp.
3. Quisque exstimavit ipsum nomen Caesaris magno terrori barbaris futurum esse.
-Every one believed that Caesar's very name would be of great terror to the savages.
4. Primá luce idem exercitus proelium acre commisit, sed gravia suorum vulnera magnae curae imperatori erant.
-At dawn the same army joined the eager battle but the severe wounds of his men were of great trouble to him.
5. Rex respondit amicitiam populi Romani sibi ornamento et praesidio debere esse.
-The kind responded that the friendship of the Roman people ought to be as an ornament and guard for him.
6. Quis praeerat equitatui quem auxilio Caesari socii miserant?
-Who was in command over the cavalry whom the allies had sent as help for Caesar.
7. Aliquibus res secundae sunt summae calamitati et res secundae sunt miro usui.
-To some fortune is of the greatest disaster and adversity is of wonderful advantage.
8. Gallis magno ad pugnam impedimento erat quod equitatus a dextro cornu premebat.
-It was of great hindrance towards the battle for the Gauls because the cavalry was pressing hard on the right wing.
9. Memoria pristinae virtutis non minus quam metus hostium erat nostris magno usui.
-The memory of old courage which was greater than the fear of the enemy was of great advantage to us.
10 . Tam densa erat silva ut progredi non possent.
-So dense was the forest that they could not advance.
II. 1. I advise you to give up the plan of making war upon the brave Gauls.
Ut omittas consilium te moneo Gallis fortibus belli inferendi.
2. Do you know where the cavalry has chosen a place for camp.
-Scisne ubi equitatus locum castris delegerint?
3. The fear of the enemy will be of great advantage to you.
-Metus hostium magno vobis erit usui.
4. Caesar left the three cohorts as a guard to the baggage.
-Caesar cohortes praesidio impedimentis tris reliquit.
5. In winter the waves of the lake are so great that they are of a great hindrance to ships.
-Fluctús hieme lacús tam magni ut magno sint navibus impedimento.
6. Caesar inflicted severe punishment on those who burned the public buildings.
-Caesar supplicium de illis aedificia publica incendentibus sumpsit.
(or you could do qui+pluperfect, I needed context)
Am I right now in saying that the Dative of Purpose is often translated into English as "as" of "of" then often of a second thing affected "to/for"?
Is there anything else that I should know? I know the exercises here are easy and so when I read hard material I fear that I may fail.