Before the lecturer entered the hall, we … the board.

Step 1: Analyze the question The sentence is: "Before the lecturer entered the hall, we .......... the board." We need the correct verb tense to fill the gap.

Step 2: Explain the concept When two past actions are described and one happened earlier than the other, we use:

• Simple past for the later action
• Past perfect (had + past participle) for the earlier action

The word "before" is a strong clue. The action that happened first (cleaning the board) takes the past perfect, and the action that happened second (the lecturer entering) takes the simple past.

Step 3: Apply it step by step

1. "The lecturer entered" is in the simple past, so it is the later event.
2. Cleaning the board was completed before that, so it is the earlier event.
3. The earlier past action needs the past perfect: had cleaned.

Checking the other options:

• A. are cleaning is present continuous and does not fit a past event.
• B. have cleaned is present perfect and does not match the past time of "entered".
• C. clean is simple present and is also wrong for a past context.
• D. had cleaned is past perfect, which correctly shows an action completed before another past action.

Completed sentence: "Before the lecturer entered the hall, we had cleaned the board."

Tip: When you see "before" with a simple past verb in one clause, look for had + past participle in the other clause. Remember: earlier action = had done, later action = did.