Final answer:
The correct subject pronoun to replace 'John and I' is 'we,' as it is used for compound subjects that include the speaker.
Step-by-step explanation:
The subject pronoun used in place of 'John and I' is we. When you have a compound subject that includes yourself and another person, you use the subjective case pronoun we. This is because the pronoun 'we' functions as the subject of the sentence, similar to how 'I' is used when it's just referring to oneself.
If 'John and I' were the objects in a sentence, then the correct pronoun would be 'us', which is the objective case. An example of using 'we' correctly is, 'John and I are going to the store', which becomes 'We are going to the store'.