consigo vs con ella

Question

In the quiz, the sentence was :  Vas a ir con ella al teatro.  I know that it doesn't sound quite right to say "Vas a ir consigo al teatro", but why would that be incorrect?

Accepted Answer

Hola Steven
The third-person personal pronouns "él", "ella", "usted", "ellos", "ellas" and "ustedes" have two possibilities when combined with the preposition "con" (with):
1. "Con él", "con ella", "con usted", "con ellos", "con ellas" and "con ustedes": The subject of the sentence is different from the pronoun that accompanies the preposition "con".
Marta vive con Lucas. Marta vive con él. (Laura lives with Pedro. Laura lives with him.)
The subject, Marta (she), is different from the complement that accompanies the preposition "con" (Pedro, he).
Mi abuela vive con mis tíos. Mi abuela vive con ellos. (My grandmother lives with my uncles. My grandmother lives with them.)
The subject, my grandmother (she), is different from the complement that accompanies the preposition "con" (my uncles, they).
2. "Consigo": The subject of the sentence is the same as the pronoun that accompanies the preposition "con."
Sara se enfadó con ella misma por haber perdido las llaves. Sara se enfadó consigo por haber perdido las llaves. (Sara got angry with herself for losing the keys. Sara got angry with herself for losing the keys.)
The subject Sara (she) is the same as the complement that accompanies the preposition "con" (herself, she).
In these cases, we usually add "mismo/a/os/as" for emphasis: 
Sara se enfadó consigo misma por haber perdido las llaves. (Sara got angry with herself for losing the keys.)
Your example "vas a ir consigo al teatro" is incorrect because the construction "consigo" is not used in that way in Spanish. To express that someone is going to the theatre with another person, the appropriate form with the corresponding pronoun is used, for example, "vas a ir con él al teatro" or "vas a ir con ella al teatro", depending on the gender of the person you are going with.
The construction "consigo" is more commonly used in contexts where a person is performing an action for themselves or carrying something with them.
In summary, "consigo" is used for actions done by oneself for one's own benefit, while "con ella" refers to the company or presence of a third person (in this case, a woman).
I hope this explanation helps you understand better.
Hasta pronto
Silvia