Questions & Answers about Я никогда не запирал эту дверь намеренно.
If I remove никогда не and just say Я запирал эту дверь, does it mean the door is locked right now?
No, it actually implies the opposite! This is the "annulled result" effect of the past imperfective. Because you used the imperfective запирал instead of the perfective запер, it tells the listener that you locked the door in the past, but the result was later undone—meaning the door is currently unlocked. (Using the perfective запер would mean it is still locked).
Since locking a door usually has a clear, completed result, why is the imperfective запирал used in this specific sentence?
When you state that an action never happened using никогда не, you almost always use the imperfective aspect. You aren't focusing on a specific, single occasion where a result failed; instead, you are making a "general factual" statement that this action simply never existed in your past. The imperfective is used to state the bare fact of the action's absence.
What is the perfective pair for запирать, and how do you form its past tense?
The perfective verb is запереть. Its past tense forms are tricky because the stem changes and drops a vowel: запер (masculine), заперла (feminine), заперло (neuter), and заперли (plural). If you successfully locked the door and it was still locked, a male speaker would say: Я запер эту дверь.
The word намеренно (intentionally) feels a bit formal. What are some more conversational alternatives?