In the sentence, "centurio nos rogavit ut captivos custodiremus," what is the use of the subjunctive. It's not a cum clause, indirect question, or result clause. That leaves purpose and indirect command. They would be translated as "we asked the centurion in order that we might guard the prisoners," or "we asked the centurion that we should guard the prison," respectively. I think it's an indirect question based on the "rogavit," but then the translation is weird. Please help.
This is an indirect command (entirely separate from indirect questions). The key is the word ut, which indirect questions don't use.
Ut in Latin can introduce four types of clauses: purpose, result, fear, and indirect command.
- Purpose clauses indicate why the main verb happens, and are the most common.
- Result clauses are only found with "so"-words (so big, so great, so small…) and indicate what happens because of the main verb.
- Fear clauses are only found with verbs of fearing and indicate what the subject is afraid of.
- Indirect commands are only found with verbs of commanding or requesting and indicate what the subject wants the object to do.
All of these can be translated as "that" (though the meaning is inverted in a fear clause, for historical reasons):
- I went to Rome so that I could see the Curia.
- The fire was so big that it destroyed half the city.
- I am afraid that you won't come back.
- I'm requesting that you do this for me.
In this case you have the verb rogāvit, "he asked"—this is third person singular, and centuriō is nominative, so it must be the centurion doing the asking. And since this is a requesting verb, it must be an indirect command: the ut-clause is what he wants the object (nōs) to do.
I would translate the whole sentence literally as:
The centurion made a request of us, that we might watch over the prisoners.
Or in idiomatic English:
The centurion asked us to watch over the prisoners.
The subjunctive use per se reveals how in the Roman mind facts in the word separated from opinions, inferences and other things, the unifying theme being these occurring in the mind of the speaker. The ut construction shows that very clearly. Rogavit here expresses the fact: the commander in fact requested something, the event of requesting did in fact happen. But the thing he is ordering is happening only in the mind yet: at the time of his request no one is guarding the prisoners, and the goal he is setting is passed on from his mind to the minds of those whom he's ordering to do the guarding. Therefore, the action of custodio is, at the moment of his speech, is still imaginary, therefore the subjunctive. Granted, in the typical use of the ut/ne constriction, the thing ordered is not yet a fact of the world, so they almost always subjunctive. This development reflects the parent PIE speech optative mood, expressing something desired or chosen by the speaker as a course of action, but not yet carried out (or with the focus on the act of choosing, not its performance). He orders that the captives must be guarded--you see already, the verb must is modal: English is also using a different mood for this. "I order the prison be guarded" is the English subjunctive, now less common than a century ago, but still fancied by purists like your truly :). So this construction has nearly grammaticalized the use of subjunctive, but the process has not been yet set in stone by the classical times, and you'll find ut with indicative in classical corpora, where the emphasis is on the devised action having been already carried out in the real world, i. e. such that has became a fact in reality to a larger degree than being just a thought, or when the writer is emphasizing the action itself.
School-level grammars usually talk very little about the subjunctive being modal in itself, and focus on its typical uses presented as hard and fast rules; this, in my opinion, later becomes one of the things that are important to unlearn when advancing to the higher level of developing the feel of what the live Latin language really is.