If I understood correctly, they are both correct but they do have different meanings as well as being spelled differently. The infinitive verb form is entender
Eu não entendo
means "I don't understand." This is in the present tense with the tonic syllable (very important & underlined) on the second to last
Eu não entendi
means "I didn't understand." This is in the past tense with the tonic syllable (still important) on the last
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