The phrase "is fastened" can be replaced with several other synonyms, depending on the context. For example, if referring to a piece of clothing that has been securely attached, one could use "is secured" or "is fixed." If talking about a lock that has been engaged, one could use "is locked" or "is latched." Alternatively, if referring to a button that has been fastened, one could use "is buttoned" or "is clipped." In any case, choosing the right synonym can add clarity and specificity to one's writing or speech.