An infinitive in Greek can be adverbial, adjectival, or substantival.  Study BBG chapter 32 or this video.  The infinitive is identified by its own unique set of endings:
| λυω | 
 present  | 
|||
| 
 λυειν  | 
 λυσαι  | 
 λαβειν  | 
 λελυκεναι  | 
|
| 
 λυεσθαι  | 
 λυσασθαι  | 
 λαβεσθαι  | 
 λελυσθαι  | 
|
| 
 λυεσθαι  | 
 λυθηναι  | 
 γραφηναι  | 
 λελυσθαι  |