"I must go."
Translation:Ik moet gaan.
The version you're proposing is grammatically incorrect, because moeten only is combined with bare infinitive verbs. That's because it's a modal verb.
Think of what happens in English with 'must': it's always combined with a bare infinitive.
Of course, there are probably exceptions, as there are in English (think of 'ought': I ought to be on time, or 'have' when meaning obligation: 'I have to pay my bills before the 10th).
Hope this helps :)