"Twas" is an old-fashioned contraction of "it was" or "it is." The word is used to convey a sense of antiquity or poeticism to a sentence. There are several synonyms for "twas" that can be used in place of the contraction, such as "it was," "it is," "it hath been," "it hath," "it's," "it has," and "it's been." Each of these words can be used interchangeably with "twas" to convey the same meaning to a sentence. When writing in a more modern style, it may be better to avoid the use of "twas" altogether, as it can come across as archaic or pretentious.