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 syllable.
entendo
Eu não entendi means "I didn't understand." This is in the past tense with the tonic syllable (still important) on the
last syllable.
entendi
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