A purpose clause is a kind of subordinate clause, and it does exactly what it says. It expresses purpose. In English, we use the phrase in order to, or to. So “I’m going to the shops in order to get milk,” or just “I’m going to the shops to get milk.”
There’s a negative version too. This is easily remembered by using the old-fashioned word lest in English: “I’m going inside lest I get soaked.” If you don’t want to sound like an 18th century dandy, you can use in order not to or so I don’t.”I’m going inside in order not to get soaked.””I’m going inside so I don’t get soaked.” But it doesn’t matter how you translate, as long as you catch the element of purpose.
Purpose clauses in Latin are easily recognised. There is a simple formula: ut + subjunctive for purpose, with ne + subjunctive for the negative version. You may have seen ut with the indicative, where it means as. Its real work is done with the subjunctive, in purpose clauses like these.