1. The dog sits under my desk.

2. The dog is sitting under my desk.

(What is the difference between Sentence 1 and Sentence 2? What is happening in #1? Is the dog under the desk now or not? I think in Sentences 2, the dog is under my desk. What about in Sentence 1?)

The dog is currently sitting under the desk in 2, and 1 only indicates that he has the habit of doing so, although it could be happening currently.

In Sentence 1, "The dog sits under my desk," it indicates a habitual or general action. It suggests that the dog has a tendency to sit under your desk, but it doesn't specify whether the dog is currently under your desk or not.

On the other hand, in Sentence 2, "The dog is sitting under my desk," it describes the current action of the dog. It means that at the time of speaking, the dog is actually sitting under your desk.

To determine which sentence is being referred to, you need to look at the context or the specific time when the statement was made. If you are talking about a regular behavior of the dog, Sentence 1 would be more appropriate. If you want to express what the dog is doing at the present moment, Sentence 2 would be more suitable.