"How many students are studying in the library?"
Translation:¿Cuántos estudiantes estudian en la biblioteca?
...which is only used if you are actually in the library at this moment studying. We use the English present continous for ongoing things that we are doing even if we are not doing them at this moment and even for the near future - both of which uses require the Spanish simple present. https://www.thoughtco.com/ways-spanish-english-verb-tenses-differ-3079929
that's not what I'm talking about. as far as I know it's either el presente del indicativo "estudian" or el presente continuo "están estudiando". Brandon asking for "están estudian" is a mix of them both.
Yes, but I added for Brandon why the Spanish present progressive is not necessarily used to replace the English present continuous.
They use the Spanish simple present much more often then we use our English simple present. So there is not a one-to-one correspondence. This translation is correct. https://www.thoughtco.com/ways-spanish-english-verb-tenses-differ-3079929
How a continuous tense is converted means what do we have to add in the root verb?
No, we cannot translate word by word. We translate the use of the English present continuous tense here with the use of the Spanish simple present. https://www.thoughtco.com/ways-spanish-english-verb-tenses-differ-3079929