Complete the astronaut's pre-flight checklist by choosing the correct verb form.

"Make sure to securely fasten your helmet before the cabin ______ depressurized."

The correct answer is is.

Just like with "as soon as" or "when," adverb clauses of time starting with before use the present tense to talk about future events. Even though the cabin will be depressurized in the future, we must use the simple present "is" in the time clause.