Questions & Answers about Это кот, и это мышь. Где она?
Why does the second sentence use она ("she") to talk about a mouse instead of an "it" pronoun?
In Russian, pronouns must match the grammatical gender of the noun they replace. The word for mouse, мышь, is a feminine noun. Because of this, you must use the feminine pronoun она to refer back to it, even though we usually say "it" in English.
Since мышь ends in a soft sign (-ь), is there a rule to know that it is feminine?
Unfortunately, you cannot tell just by looking at it! Nouns ending in a soft sign in their dictionary form (nominative case) can be either masculine (like день for "day") or feminine (like мышь for "mouse"). You simply have to memorize the gender when you learn a new soft-sign noun.
If мышь is feminine and кот is masculine, why does the sentence use Это for both?
When you are pointing something out and saying "This is a...", you always use the fixed, default form Это, regardless of the noun's gender. You only need to match the gender if you are saying "this specific object", such as эта мышь (this mouse) or этот кот (this cat).
Why is there a comma before и (and)?
In English, we often drop the comma before "and" in short sentences. However, Russian punctuation rules are stricter: if и connects two complete mini-sentences that each have their own subject (here, "this is a cat" and "this is a mouse"), you must use a comma.