Below is the text from the lesson explaining your options with "por si/por si acaso". All of the examples use the imperfect subjective, but the fact that was the only option wasn't really clear until I read through the comments/questions.
From the Lesson: They introduce a subordinate clause expressing a condition. They can be followed by the subjunctive or the indicative.
Sign in to submit your answer
Don't have an account yet? Join today
Test your Spanish to the CEFR standard