Need help with ここ and これ


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」




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.)
>

