What is a purpose clause and how do you construct one in Greek?

A purpose clause is a sentence that expresses purpose - doing something in order that something else might happen. For example, 'he shouted so that the girl would help him', 'I went towards Athens to see the king', 'they came inside in order not to be caught in the rain'. There are two main ways of forming these in Greek. One is ἱνα + subjunctive (e.g. ἐβοησεν ἱνα ἡ κορη βοηθῃ = he shouted so that the girl would help him), and the other is ὡς + future participle (προσηλθον Ἀθηναζε ὡς ὀψομενος τον βασιλεα = I went towards Athens to see the king). Unfortunately for you, both of these could come up in the GCSE Greek exam, but fortunately they're both quite straightforward.