The imperative mood is the mood used to give a command.  Study BBG chapter 33.
I went to bed at 10pm.
 Go to bed at 10pm.
Note the second sentence is a command. Hence, the verb “go” is in the imperative mood. The verb “went” is in the indicative mood.
The imperative mood can also be used in the following ways:
Command – from superior to inferior
Prohibition – μη + imperative
Request – from inferior to superior
Full Paradigm:
| 
 Present Active 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 λῦε 
 | 
 λύετε 
 | 
| 
 3rd person 
 | 
 λυέτω 
 | 
 λυέτωσαν 
 | 
| 
 Present Middle/Passive 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 λύου 
 | 
 λύεσθε 
 | 
| 
 3rd person 
 | 
 λυέσθω 
 | 
 λυέσθωσαν 
 | 
 
| 
 Aorist Active 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 λῦσον 
 | 
 λύσατε 
 | 
| 
 3rd person 
 | 
 λυσάτω 
 | 
 λυσάτωσαν 
 | 
| 
 Aorist Middle 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 λῦσαι 
 | 
 λύσασθε 
 | 
| 
 3rd person 
 | 
 λυσάσθω 
 | 
 λυσάσθωσαν 
 | 
| 
 Aorist Passive 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 λύθητι 
 | 
 λύθητε 
 | 
| 
 3rd person 
 | 
 λυθήτω 
 | 
 λυθήτωσαν 
 | 
 
| 
 2nd Aorist Active 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 βάλε 
 | 
 βάλετε 
 | 
| 
 3rd person 
 | 
 βαλέτω 
 | 
 βαλέτωσαν 
 | 
| 
 2nd Aorist Middle 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 βαλοῦ 
 | 
 βάλεσθε 
 | 
| 
 3rd person 
 | 
 βαλέσθω 
 | 
 βαλέσθωσαν 
 | 
 
| 
 Present Imperative of ειμι 
 | 
 | 
 Singular 
 | 
 Plural 
 | 
| 
 2nd person 
 | 
 ἴσθι 
 | 
 ἔστε 
 | 
| 
 3rd person 
 | 
 ἔστω 
 | 
 ἔστωσαν 
 |