A Question for Your Class on Acts 16
When the Holy Spirit forbid the Apostle Paul to speak the word in Asia, the Holy Spirit opened another door for him to speak the word elsewhere.
You might consider asking your class if they can remember a time when a door was shut to them, and then a different (perhaps better) door was opened for them. You might ask them to share their experience with the class. It is possible too that the door that was opened to them blessed others as well as them, even as the new open door enabled the Holy Spirit to work through Paul and lead Lydia to faith in Jesus Christ.
Sometimes a new door is opened for us that means we must shut a door behind us and move to a new place of ministry. Perhaps as a matter of time and place, we cannot keep both doors open. Perhaps your Bible teaching has meant you had to rearrange your schedule. The "open door" meant closing a door.
For example, about a year after I began the online International Bible Lessons Commentary, I believe the Holy Spirit led me to close a door so I could work full time in study and writing the International Bible Lessons Commentary. Time and reader responses have shown me that was the right decision, which are extra special confirmations from the Lord.
You might want to share with your class one of your personal experiences as an example. You might encourage other teachers and Bible students by sharing your experience here on the International Bible Lessons Forum.