In French, there are 2 main past tenses - the passé composé, and the imparfait. The passé composé is used when an action has been completed, for example, when in English we would say 'I ate' or 'I have eaten.'
The imparfait is used when an action is incomplete, when in English we would say 'I was eating' or 'I used to eat'.
This is formed with the auxiliary verb (either avoir or etre) and the past participle. You just have to learn what the past participles are, but usually the infinitive minus the ending, plus an different ending to make it the past participle. For instance:
'Manger' (to eat) minus ending ER = 'mang,' which doesnt mean anything. We need to add an ending to make it a past participle, so we here add an 'é,' making 'mangé' - and voilà, the past participle. Bear in mind there are different rules for verbs with different endings.
To make use this in the context of the passé composé, use 'avoir' as an auxiliary verb. Let's take the 'je' form to make it simple. So, the je form of avoir would be 'j'ai' meaning, 'I have'. Then add in the past participle: 'j'ai mangé' - literally, 'I have eaten.' But in French there is no distinction between 'I ate' and 'I have eaten,' so you use this tense for both situations. For different people, use different forms of the verb, so 'she ate' would be 'elle a mangé' and they (male) would be 'ils ont mangé.'
Sometimes, 'etre' will be the auxiliary verb, and this is used for the following verbs
Monter - past participle: monté (went up)
Retourner - past participle: retourné (returned)
Sortir - past participle: sorti (went out)
Venir - past participle: venu (came)
Arriver - past participle: arrivé (arrived)
Naître - past participle: né (was born)
Descendre - past participle: descendu (went down)
Entrer - past participle: entré (entered)
Rester - past participle: resté (stayed)
Tomber - past participle: tombé (fell)
Rentrer - past participle: rentré (went back in)
Aller - past participle: allé (went)
Mourir - past participle: mort (died)
Partir - past participle: parti (left)
This is formed differently with ER, IR, and RE verbs. We will take an example of each to show you how they are formed (bear in mind there are some exceptions to these rules, however!)
Lets take 'manger' as an example again. For this all imperfect verb foms, you take the nous form of the present tense, in this case making 'mangeons,' and take off the -ons, leaving 'mang'. Then, add different endings depending on the person. The endings for different people are as follows:
je = ais --> used with manger: 'je mangais' tu = ais --> used with manger: 'tu mangais' il = ait --> used with manger: 'il mangait' nous = ions --> used with manger: 'nous mangions' vous = iez --> used with manger: 'vous mangiez' ils/elles = aient --> used with manger: 'ils mangaient'
Let's take 'finir' as an example. The way you construct the verb is the same as 'manger,' but the 'nous' form is different because it is an 'IR' verb: nous finissons. Take off the -ons, like we did before, which leaves 'finiss.' Then, add the same endings:
je = ais --> used with manger: 'je finissais' tu = ais --> used with manger: 'tu finissais' il = ait --> used with manger: 'il finissait' nous = ions --> used with manger: 'nous finissions' vous = iez --> used with manger: 'vous finissiez' ils/elles = aient --> used with manger: 'ils finissaient'
This is again the same process - the nous form of the verb, plus the endings. We'll take the verb 'comprendre' as an example. The 'nous' form is 'comprenons,' so without the 'ons,' the stem will just be 'compren'. Then, we can add the endings.
je = ais --> used with comprendre: 'je comprenais' tu = ais --> used with comprendre: 'tu comprenais' il = ait --> used with comprendre: 'il comprenait' nous = ions --> used with comprendre: 'nous comprenions' vous = iez --> used with comprendre: 'vous compreniez' ils/elles = aient --> used with comprendre: 'ils comprenaient'
So, there we have it, a comprehensive description of the differences between the two main past tenses and how to use them.