When your teacher said "check your vosotros form endings and don't forget the accents!" it is because in first conjugation verbs the pronoun "vosotros/as" (third person plural) is always stressed.
The ending of the pronoun "vosotros/as" ends in "-áis", that is why the hiatus must be separated with a tilde.
The accents, in Spanish, are a mark that is placed on top of the letter to mark greater intonation.
MissSpanish