Learn Dutch: Ik schrok erg van zo'n harde klap. - I was very startled by such a loud bang.

Zo'n is short for zo een and translates to "such a". You use it here because klap (bang) is a singular, countable noun. If the sentence was about multiple bangs ("such loud bangs"), you would use zulke: zulke harde klappen.

Why does the adjective hard get an -e added to it after zo'n?

Zo'n follows the exact same adjective rules as the word een (a/an). Because klap is a de-word (de klap), any adjective between een or zo'n and the noun gets an -e ending. If it were a het-word, there would be no -e (for example: zo'n hard geluid).

The English translation says "startled by", but the sentence uses van instead of door. Why?

The Dutch verb schrikken (to be startled/frightened) takes a fixed preposition: van. While English speakers are startled by something, Dutch speakers are always startled from something (schrikken van).

Does harde mean "loud" or "hard" here?