A purpose clause is a clause that shows the purpose of the action of the main clause of a sentence. In the sentence “I ran to the store to buy some cocktail wienies,” for example, “I ran to the store” expresses a fact — something that happened — so in Latin it would be in the indicative mood. The “to buy cocktail wienies” part, however, tells why “I ran to the store.” It doesn't say whether or not I actually bought any wienies. Since buying wienies was the idea behind the trip, in Latin it would be cast into the subjunctive mood.

Let's take a closer look at the way English expresses purpose. It is important to understand how the English construction works, because Latin takes a different tack.

In English, the main clause is independent, but the purpose clause isn't even a clause (a phrase with both a subject and a verb)! English uses an infinitive phrase instead. Latin never uses an infinitive to express purpose. You could use a construction in English that more or less parallels the Latin approach to expressing purpose, but it sounds awkward: “I ran to the store in order that I might buy cocktail wienies.”

The Latin version has two clauses: Both the first and the second part have subjects (via personal endings) and predicates. The first part is the main clause. Its verb is in the indicative mood because it relates a fact. The second part, the subordinate clause, has a verb in the subjunctive because it doesn't describe a fact. It illustrates a wish or idea.

The subordinating conjunction that introduces a purpose clause in Latin is the word ut. However, if the purpose clause is negative it is introduced by the word nē instead.

Once again, Latin has two clauses, a main, independent one, then the subordinating conjunction nē, and finally a subordinate clause in the subjunctive. As far as the grammatical constructions are concerned, the only difference between a positive versus a negative purpose expression in Latin is the use of nē instead of ut.

The English, on the other hand, does a flip-flop. In the earlier example of a positive purpose expression, there was an infinitive phrase (“to buy …”) and the subordinated clause version sounded awkward (“in order that I might buy …”). This time, with a negative purpose expression, English uses a subordinate clause. You could use an infinitive phrase, but it sounds funny: “I ran to the store for us not to be hungry at the party.”

Latin is consistent and logical. English isn't. In fact, there are many ways to translate purpose clauses into English. When you become more familiar with the gist of the Latin, you will be able to find many ways that sound better than the suggestions offered above.