Questions & Answers about Usta će utrnuti posle injekcije.
Because usta is one of those Serbian nouns that is grammatically plural even when it refers to one mouth. In English we translate it as singular: mouth.
So:
- usta = mouth
- but grammatically it behaves like a plural form
That is normal in Serbian. In this sentence, you do not really notice the agreement because the future is formed with će + infinitive.
Here it means mouth.
If you specifically want to say lips, the usual word is usne.
So:
- usta = mouth
- usne = lips
In a dental or medical context, Usta će utrnuti is a natural way to say that the mouth area will go numb.