There are two basic past tenses in French.
The passé composé is used if something happened just once.
The imparfait is more if something "was happening" over a period of time.
Conjugating the passé composé:
Most verbs use avoir, but some use être.
The verbs which use être are: reflexive verbs (se ~) and "vandertramps" (there is a song you can look up to learn these, there are only 16)
Rugir is neither of these, so it uses avoir.
You also need to form the past participle.
Take off the ending of the verb (er, re, ir) and add the appropriate suffix (é, u, i)
(some verbs have irregular past participles, you'll learn them later.)
In this case, we take the ir off rugir to get rug- as our stem, and then get rugi.
If you use être, you must conjugate the past participle to the gender/number of the subject, but we are not.
Le lion a rugi.
Conjugating the imparfait:
Take the present tense nous form of the verb and remove the -ons
(There's only one irregular imparfait verb, être, which uses the stem ét-)
Add the appropriate suffix for the subject
je ⇒ ais...tu ⇒ ais...il/elle/on ⇒ ait.
nous ⇒ ions...vous ⇒ iez...ils/elles ⇒ aient.
Le lion rugait.