Aorist verbs are distinguished into first aorists and second aorists (study chapter 22&23 in BBG). This difference is the same as the difference between English regular and irregular verbs. For example, note the difference in the way the following verbs form the past tense:
Today, I walk; yesterday, I walked.
This is the regular way past tenses are formed in English. The suffix -ed is added to the verb. This corresponds to the first aorist in Greek. Now consider the English verb to hold:
Today, I hold the baby; yesterday, I held the baby.
Note the past tense of “hold” is not “holded”. Instead of adding -ed to the verb, the spelling of the verb changes. The verb “hold” is an irregular verb, and this corresponds to a second aorist is in Greek.
First aorist verbs (or “regular aorists”) are identified by the tense formative σα (corresponds to the -ed suffix on English regular verbs).  An easy way to know if a given verb is first or second aorist is to find the verb in MBG.  Then check and see if the stems are spelled the same in all the different forms.
| λυω I loose  | 
present tense | 
| ελυσα I loosed  | 
 aorist tense Note: the stem (in red) is spelled the same in both the present and aorist tense.  | 
| ἀκούομεν we hear  | 
present tense | 
| ἤκουσαμεν we heard  | 
aorist tense Note: the stem (in red) is spelled the same in both the present and aorist tense. The first letter changed because of the augment.  | 
.
Second aorist verbs (or “irregular aorists”) don’t take a tense formative.  Instead, they change their spelling.  λαμβανω is an example of such a verb.
| λαμβανω I receive  | 
present tense | Note: the stem here is λαμβαν- | 
| ελαβον I received  | 
aorist tense | Note: the stem has changed to λαβ- | 
| ερχομαι I come  | 
present tense | Note: the stem here is ερχ- | 
| ηλθον I came  | 
aorist tense | Note: the stem has changed to ηλθ- | 
Here you can see the present forms next to the aorist forms:
| Person | Present Active | Aorist Active | 
|---|---|---|
| 1st Singular | λαμβάνω (I receive) | ἔλαβον (I received) | 
| 2nd Singular | λαμβάνεις (you receive) | ἔλαβες (you received) | 
| 3rd Singular | λαμβάνει (he/she/it receives) | ἔλαβεν (he/she/it received) | 
| 1st Plural | λαμβάνομεν (we receive) | ἐλάβομεν (we received) | 
| 2nd Plural | λαμβάνετε (you receive) | ἐλάβετε (you received) | 
| 3rd Plural | λαμβάνουσι (they receive) | ἔλαβον (they received) | 
.
.
Aorist Indicative:
| Person | Active Voice | Middle Voice | Passive Voice | 
|---|---|---|---|
| 1st Singular | ἔλαβον | ἐλαβόμην | — | 
| 2nd Singular | ἔλαβες | ἐλάβου | — | 
| 3rd Singular | ἔλαβε(ν) | ἐλάβετο | — | 
| 1st Plural | ἐλάβομεν | ἐλαβόμεθα | — | 
| 2nd Plural | ἐλάβετε | ἐλάβεσθε | — | 
| 3rd Plural | ἔλαβον | ἐλάβοντο | — | 
Aorist Imperative:
| Person | Active Imperative | Middle Imperative | Passive Imperative | 
|---|---|---|---|
| 1st Singular | — | — | — | 
| 2nd Singular | λαβέ | λαβοῦ | — | 
| 3rd Singular | λαβέτω | λαβέσθω | — | 
| 1st Plural | — | — | — | 
| 2nd Plural | λάβετε | λάβεσθε | — | 
| 3rd Plural | λαβέτωσαν | λαβέσθωσαν | — | 
Aorist Subjunctive:
| Person | Active Subjunctive | Middle Subjunctive | Passive Subjunctive | 
|---|---|---|---|
| 1st Singular | λάβω | λάβωμαι | — | 
| 2nd Singular | λάβῃς | λάβῃ | — | 
| 3rd Singular | λάβῃ | λάβηται | — | 
| 1st Plural | λάβωμεν | λάβωμεθα | — | 
| 2nd Plural | λάβητε | λάβησθε | — | 
| 3rd Plural | λάβωσι(ν) | λάβωνται | — |