Formal:
* 'Twas: This is the archaic contraction, often used in poetry or formal writing.
* It's: This contraction is used more commonly in informal writing, but can also be used in formal writing if it fits the tone.
Informal:
* Was: This is the most common way to shorten "it was" in casual conversation.
* 't was: This is a more informal contraction of "it was," often used in casual speech.
* It'd: This is the contraction of "it would," which can sometimes be used to shorten "it was" depending on the context.
Example:
* It was a beautiful day.
* 'Twas a beautiful day.
* It's a beautiful day.
* Was a beautiful day.
* 't was a beautiful day.
* It'd be a beautiful day if it wasn't for the wind.
The best way to shorten "it was" depends on the context and desired level of formality.