Supplemental to Colin Fine's answer:
The distinction between the English infinitive and base present forms is usually invisible. In grammatical analysis, however, the distinction must be drawn, because there is one English verb for which the two forms differ: the very common verb BE. Consider your sentences with BE instead of GO:
She must be there. ... She must *are there is unacceptable.
I am there. ... I *be there is unacceptable (though it does occur in some dialects).
We therefore distinguish the base form of other verbs as infinitives when they are used as complements of modal auxiliaries.
Analysis gets a little trickier with situations where the infinitive/base form appears to be employed as a finite verb. Are these uses of be and go infinitives or something else?—
Be honest! Go away!
We demand that he be honest. We demand that he go away.
- And there's also the conditional use; it's obsolete now, but you're likely to encounter it in older texts:
If he be honest we will succeed. If he go now we are lost.
I'm inclined to see these as different uses of a single form; but in your studies you will probably be safer following traditional analysis and treating them as distinct 'imperatives' and 'subjunctives'.