Dear N T:

You wrote:

 1-We only did not open the front door.

Is this sentence correct and does it mean:

We did everything else, but we did not open the front door.

Or:

We opened all the doors except the front door.

This totally depends on context. Either makes sense. You are saying that the one action you did not do was open the front door. If the context is talking about opening doors, then the second one would fit. If the context is speaking of a number of activities then the first one could make sense. What this says is “The one thing we did not do was open the front door.”

If you think about it, “only” as an adverb is the adverb form of  ‘one.”

2-We did not open the front door only.

Is this sentence correct and does it mean:

We opened doors other than the front door as well.

Since this follows “front door,” that is what “only modifies.” This would mean that other doors were opened, too.

Usually when a sentence ends with “only,” that add emphasis. You could also say “We did not open only the front door.”