Write the correct form of the word TO BE to fill in the gap: A lion ___ a big cat.

The correct form of "to be" to fill in the gap is "is". So, the completed sentence is: A lion is a big cat. **Explanation:** In English, the verb "to be" has different forms depending on the subject. For singular subjects (like "a lion"), we use "is" in the present tense. Other forms include: * **am**: used with the subject "I" (e.g., "I am a student"). * **are**: used with plural subjects (e.g., "They are friends") or the subject "you" (e.g., "You are kind").