A grammar question.
 

Hi all.

I saw this line of sentence in the textbook I have been using to teach myself:
犬は箱の中にいます。

Would：
箱の中に犬がいます。

... be the same?

Some of the previous example in the book uses〝[location]＋に＋[noun]＋が＋います　or　あります”, so I am wondering if this rule applies to the above sentence too.

Good question.

Both are completely grammatical but there is a rather huge nuance difference. The two sentences are NOT interchangeable.

犬は箱の中にいます。= "The dog is in the box."
The topic is the dog. The dog has already been mentioned in the conversation. The sentence can also be used as the answer to the question "Where is the dog?"

箱の中に犬がいます。= "There is a dog in the box."
We would use には more often than just に. The topic is the box or inside the box. The dog has NOT been mentioned in the conversation. The sentence can also be used as the answer to the question "What is in the box?"

That's a very detailed and informative explanation. Thanks a lot! :)

These kind of problems comes up quite often since I'm studying another language by myself, and even language learning softwares would not be able to provide me with full support.