Double verb conjugation in a sentence

I would like to raise a question

I was advised by a couple of different sources that only a the first verb in a sentence needs to be conjugated. I think there were exceptions such as negations or interrogatives but i have the following sentence and i believe the second very - compreender - can stay as an infinitive

It’s possible this is considered as two statements in which case in concede but I believe the following sentence is correct

Eu falo só um pouco português mas não compreender tudo

Thanks