"Opimme toisiltamme" is "we learn from each other". The words 'each other' do not always look the same, unfortunately. For example, the sentence 'we look at each other' would be translated to 'katsomme toisiamme'. Additionally, "we don't talk to each other" would be "emme puhu toisillemme". The Finnish equivalent of 'each other' changes based on the verb that precedes it. I'm afraid I don't have a more specific grammar rule at hand, but I hope this helps a little.

I can explain the grammar for you. "Each other" translates to the following construction in Finnish: Toinen ('second, other') in plural > toiset ('others') + possessive suffix. The "toiset" part is declined in the same case as any other object and then the possessive suffix (in the same person as the subject) is attached to this form. Minä opin sinuLTA. - I learn from you. Me opimme toisiLTAmme. - We learn from each other. Lastly, the case of the object depends on the verb. There's no fixed rule to follow concerning this - just consult a dictionary when in doubt.