Is the nosotros form of -ar verbs the same in present and preterite?

I don’t know how I only just realized this, but now I’m confused. Looking at a chart and for nosotros in the present they have: hablamos, comemos, abrimos. Then for preterite: hablamos, comimos, abrimos. The er verbs are the only ones that change?? How do you know which tense is being used for ar and ir if it doesn’t change?

submitted by /u/sleepwithmythoughts
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