Unlike English, the same pronouns are used for he/she/it/this in hindi. This is for someone or something close by.
yah (pronouced "yay")
For someone or something not present or far away the pronoun
vah (normally pronounced "vo")
means he/she/it/that
however, these prounouns change spelling when followed by post positions.
For example, he /she = vah, but "to him" is usko
the word "ne" is a special post position that marks the subject of a sentence when certain verbs are used in the past tense and is not translated to anything in English.
thus the translation usne kahaa means he/she said.
kahaa is the past tense of kahnaa = to say
The past tense verb conjugations agree ergatively with the direct objects of the verb. In this case since the direct object is not mentioned the conjugation defaults to the singular masculine conjugation for kahnaa in the past tense.
By the way "ergativity" is a linguistic term that you can read about in wikipedia or something. It is not seen in most European languages (if any) and hindi only has this ergativity characteristic when we are talking about past tense and transitive verbs. It is what makes hindi a rather challenging language to learn because you have to keep track of when to conjugate for subjects or objects in the sentence.
"ne" by the way is called the ergative marker here by linguists.
Anyway that is the appeal of Indo-Aryan langauges I guess.
So, how do you tell who "us" is referring to in the sentence? You have to do it by context, otherwise it could mean either he/she/it /that without contextual information.
In other tenses where "ne" is not involved, the verbs are conjugated for both gender and pluralitiy. Thus:
vah kahtaa hai = He says,
vah kahtii hai = She says
For the senses of vah that mean "it" or "that" the conjugation depends on the gender of the noun the pronoun is referring too.