Learn Farsi: آن موش داخل سوراخ رفت چون شما آن را بیش از حد ترساندید. - That mouse went inside the hole because you scared it too much.

بیش از حد literally translates to "more than the limit" and is used to mean "too much" or "excessively". In this sentence, it acts as an adverb modifying the verb ترساندید (you scared). Just like most adverbs in Farsi, it is placed immediately before the verb it modifies.

I know the verb for "to be scared" is ترسیدن (tarsidan). Why does the verb here look like ترساندید (tarsaandid)?

This is the "causative" form of the verb! If you take the present stem of ترسیدن (tars) and add ـان (-aan), you get a new verb: ترساندن (tarsaandan). This changes the meaning from "to be scared" to "to make someone scared" (to scare). ترساندید is simply the past tense meaning "you scared".

Is there an Ezafe sound connecting داخل (daakhel) and سوراخ (sooraakh)?

Yes, it is pronounced daakhel-e sooraakh. Words that show location or direction, like داخل (inside), توی (in), or روی (on), use the invisible -e sound (Ezafe) to connect to the noun that follows them.

What is آن را (aan raa) doing in the second half of the sentence?

آن usually means "that", but here it translates to "it" and refers back to the mouse. را is the direct object marker. Because "it" is a specific noun receiving the action (it is the thing you scared), Farsi requires right after it.