What is the purpose of "lo" in the following sentence?
Esta ciudad no es lo bastante grande para los dos. - This town isn't big enough for the two of us.
Could you not just say "Esta ciudad no es bastante grande para los dos".
(Sentence and translation from https://www.spanishdict.com/translate/bastante)
Neuter article "lo" can be used to nominalize adjectives, that is, to transform them into nouns.
Therefore, if we say:
Here we can read:
3.16.6. lo + adjetivo + que
Equivale a un adverbio de cantidad como cuánto, cuán, qué.
Ejemplos:
Hay que ver lo bueno que es este niño. (how good) Verás lo útil que te resulta. (how useful) Entonces es cuando me di cuenta de lo despistado que estaba. (how distracted) Me impresionó lo furioso que se puso. (how upset)
Though related to the usages above, the usage in question is a special case because there is an extra adverb before the adjective. Both sentences, with and without "lo", mean the same thing:
Other similar cases of "lo" followed by an adverb and an adjective are:
No se puede creer lo excesivamente costosa que es esta ciudad.
Lo más increíble de esta ciudad es que podemos convivir los dos.
What makes "lo" optional in the sentence at issue is that the structure continues to be adjectival. I think this only happens with "bastante" and "suficientemente":
It comes to how you form the sentence.
When you include "lo", you are saying that the city is not big enough for you both, it acts as a neutral article and is used to turn "bastante grande" into an abstract noun phrase, emphasizing the sufficiency of size.
When you don't include "lo", the sentence tends to become into a question tone, in this case it is suggesting that the city is bigger than you both thought it would be.
Sometimes, both of them are understood as the same, but for some people it would not be seen as it, which can lead to a misunderstanding.
