THE PAST HISTORIC (le passé simple)
Like the perfect tense, the past historic refers to an action of any duration completed in the past (as opposed to the imperfect tense, which indicates a continuing action in the past). The past historic is only used in its written form (never in everyday conversation), mainly in novels, short stories and newspaper articles. Very often, the past historic and the perfect tense share the same value and are interchangeable in that respect. Yet, when they are used in the same text, they convey a slightly nuanced difference as the past historic indicates that the event has no immediate relevance to the present while the use of perfect tense suggests that there is still a relevance to the present.
The endings for verbs ending in -er are -ai, -as, -a, -âmes, -âtes, and -èrent. Thus, aller would conjugate as follow: j'allai, tu allas, il alla, nous allâmes, vous allâtes, ils allèrent.
The endings for verbs ending in -ir or -re can be either -is, -is, -it, -îmes, -îtes and -irent or -us, -us, -ut, -ûmes, -ûtes and -urent.
Be careful with some verbs endind in -ir and -re as the singular forms look the same in the present tense and the past historic.
Examples: finir: je/tu finis, il finit
dire: je/tu dis, il dit
The context should help you figure out in which tense the verb is conjugated (look for example at the order of the verbs in the rest of the paragraph).
Note also that a lot of high-frequency verbs present an irregular pattern, so memorising the most important one will prove useful. Below is a short series of flashcards that will enable you to do so (they show only the 3rd person singular since it is probably the most relevant to your needs but you can easily work out the other forms by removing the final -t and adding the following ending: je: -s, tu: -s, nous: -^mes, vous: -^tes, ils: -rent)[D]