I don't understand why you're able to interchange これ in 「これはペンです」and 「ペンはこれです」, but you can't interchange ここ in 「まどはここです」 and 「ここはまどです」
2 Answers
Your assumption is not correct. In both pairs, the two sentences are not interchangeable. は is called a topic marker. Basically, it marks what you're talking about. Changing the topic of a sentence will usually change the "feeling" of the sentence drastically.
これはペンです and ペンはこれです are both grammatical, but are not interchangeable. The following two sentences are very different in meaning.
- これはペンです。 (This thing? This is a pen.)
- ペンはこれです。 (Looking for a pen? Here, this is the pen, here you go.)
Likewise, まどはここです and ここはまどです are both grammatical, but are not interchangeable. The following two sentences are very different in meaning.
- ここはまどです。 (Curious about this structure? This is a window.)
- まどはここです。 (Looking for a window? Here it is.)
I think it's because of the topic particle. In your first sentence,「これ」 and 「ペン」refer to the same thing. While「ここ」refers to a place; the subject is「まど」.
