Imperfectum is a past tense. Its formation is simple: we take the 3rd person plural present and change -ant into -abam, and both -ent, -unt into -ebam. The endings of this tense are like in praesens, except -m in the 1st pers. activi and -r in the 1st pers. passivi. Examples : I conjugation: voco, vocare, vocant:

person	singular	plural
1	vocabam	vocabamus
2	vocabas	vocabatis
3	vocabat	vocabant

II conjugation: video, videre, vident:

person	singular	plural
1	videbam	videbamus
2	videbas	videbatis
3	videbat	videbant

In the IV conjugation the 3rd person plur. ends in -iunt :

person	singular	plural
1	venio	venimus
2	venis	venitis
3	venit	veniunt

III conjugation, regular verbs:

person	singular	plural
1	mittebam	mittebamus
2	mittebas	mittebatis
3	mittebat	mittebant

III conjugation, verbs adding -i-:

person	singular	plural
1	faciebam	faciebamus
2	faciebas	faciebatis
3	faciebat	faciebant

The verb sum forms imperfectum irregularly:

person	singular	plural
1	eram	eramus
2	eras	eratis
3	erat	erant

Similarly : possum, potest > poteram, prosum, prodest > proderam, absum, abest > aberam. Other irregular verbs : ire and its forms with prefixes have -ibam : ire > ibam, abire > abibam Another exception, similar to possum, posse is: prosum, prodesse (prosum, prodes, prodest, prosumus, prodestis, prosunt).
The verb eo, ire is conjugated like a regular verb of III conjugation, but it has e- before endings -o and -unt. The other verbs form imperfectum regularly : fero, ferre, ferunt > ferebam volo, velle, volunt > volebam nolo, nolle, nolunt > nolebam malo, malle, malunt > malebam fio, fieri, fiunt > fiebam aio, aiere, aiunt > aiebam inquam has only a form of 3rd person sing. inquiebat. Imperfectum passivi has passive endings after -ba- : -bar, -baris, -batur, -bamur, -bamini, -bantur:

person	singular	plural
1	vocabar	vocabamur
2	vocabaris	vocabamini
3	vocabatur	vocabantur