Latin uses the subjunctive to express purpose. For purpose clauses, the action of the subordinate clause logically occurs after the action of the main verb. (I do X now in order that I may have Y in the future). Thus, for purpose clauses, you will find either the present subjunctive or the imperfect subjunctive used (depending upon the sequence of the main verb).
General rule for translating purpose clauses:
Primary sequence: uses present subjunctive.
I ______________ in order that I may ______________.
Secondary sequence: uses imperfect subjunctive.
I ______________ in order that I might ______________.
The subordinating conjunction ut is used for positive statements, ne for the negative.
ut = in order that ne = in order that . . . not
NOTE: There are a number of different ways to express purpose in English, thus there will be more than one correct way to translate Latin purpose clauses.
Primary sequence:
Pugnat ut nautam superet.
He fights in
order that he may overcome the sailor.
He fights to
overcome the sailor.
He fights for
the purpose of overcoming the sailor.
Pugnabit ut nautam superet.
He
will fight in order that he may overcome the sailor.
Pugnaverit ut nautam superet.
He
will have fought in order that he may overcome the sailor.
Secondary sequence:
Pugnabat ut nautam superaret.
He
was fighting in order that he might overcome the sailor.
Pugnavit ut nautam superaret.
He
fought in order that he might overcome the sailor.
Pugnaverat ut nautam superaret.
He
had fought in order that he might overcome the sailor.