Dīxit, et potentem hastam magnīs vīribus manūs sinistrae in uterum equī iēcit; stetit illa, tremēns.
Does stetit illa refer to the spear? As in "it stands there?" Is illa capable of carrying that meaning?
This passage is from Wheelock's Latin.
According to the Oxford Latin Dictionary, illa can be adverbial and mean 'there,' but only if it has a long a (illā). Since your text marks long vowels, you know it can't be this.
With a short a, the only thing it can be in your sentence is the feminine nominative singular form of the demonstrative pronoun. It refers to the hasta.
illa is referring to the spear, but it wouldn't be inappropriate to translate this clause as "There it stood, quivering". Assuming one is trying to evoke the sort of poetic English of e.g. the King James Bible, stetit could be rendered "there stood".
stetit hasta tremēns -> "There stood the spear, quivering"
Replacing the noun with its prounoun, becomes:
stetit illa tremēns -> "There it stood, quivering"
I don't know the answer. I don't know if illa could be construed as 'there,' but I haven't come across that use.
Only, it seems simpler to take illa as nominative, identifying hasta(fem.) as the implied subject of stetit, with the narrator showing that it was not the 'quivering belly (neuter)' or the 'twitching horse (masc)' or the 'shaking spear-thrower (masc)' that stood;
'...he hurled the spear, it stood quivering.'
I also vote for illa as referring to the spear for the above mentioned reasons, but also because of the switch in subject from iecit = "he threw", to stetit = "it stood". If the illa were not there, the common Latin stylistic preference for omission of explicit subject nouns/pronouns would suggest: " He stood, trembling."
Moreover, the perfect tense is used here, indicating conceptually the completion of an event, rather than an ongoing state (imperfect tense). As such, I'd prefer to render stetit as "It stuck there, trembling," giving more a sense that the spear had been thrown, was in motion, and then (completed event) stopped, i.e., came to a "standstill".