a. Before Sarahi can board the bus, shei needs to get some coins for the fare.
b. Before shei can board the bus, Sarahi needs to get some coins for the fare.
- Refer to anything in the speaker's and listener's environment that they are both aware of (they can be exophoric).
- Refer to something previously mentioned within the sentence or within the conversation.
- Refer to something higher up in the clause structure (so a pronoun in a subordinate clause can refer to a thing or person mentioned in the main, matrix clause).
Sentence (a) is fine because she co-refers backwards with Sarah which appears earlier in the sentence.
Sentence (b) is fine because she appears in a subordinate clause and refers to Sarah which is in the main clause.
Both types of sentence are perfectly acceptable and are very common in all types of speech and writing. Notice, however, that if none of conditions (1-3) hold, then she will not be able to co-refer with another common noun in the sentence. So we cannot have she in a main clause trying to co-refer with a later Sarah in a subordinate clause.
c. *Shei needs to get some coins for the bus before Sarahi can board it.
In sentence (c) she and Sarah cannot refer to the same person. This is because, firstly, she occurs before Sarah. Secondly, Sarah is subordinate, and she cannot refer to a noun in a lower clause. This sentence is not possibble.