次第|しだい

1. 1As soon as something happens, a subsequent action will follow.Ex. 1
2. 2An outcome depends on a particular factor.Ex. 2

The “as soon as” use often introduces a planned follow-up action and fits polite notices and correspondence. The “depends on” use states what determines an outcome and is not limited to polite contexts.

• 次第だ / 次第でN2次第だ／次第で is the 'depending on' sense of 次第: both attach to nouns, and in this sense they are interchangeable. 次第, however, has an additional 'as soon as' use — attaching to the ます形 with ます dropped, or to an action noun — which 次第だ／次第で does not have.
• いかんでは / いかんによってはN1いかんでは／いかんによっては is close only to noun + 次第では, and in meaning the two are basically interchangeable, but it is more formal and bookish, appearing mostly in official documents and notices; の can be added after the noun. It has no 'as soon as' meaning.
• 〜次第ではN2〜次第では corresponds only to the 'depending on' sense: it is 次第で with は added, emphasizing that a certain situation may produce an unusual — often unfavorable — result, and the second clause often pairs with かもしれない or こともある. It has nothing to do with the 'as soon as' sense.
• や否やN1や否や is close only to the 'as soon as' sense: it attaches to the 辞書形, is a written-style expression, mostly describes things that have already happened, and its second clause cannot express volition or a request. 次第 attaches to the ます形 and the like, speaks of future matters, and its second clause usually expresses volition or a request, so the two are not interchangeable.
• なりN1なり is close only to the 'as soon as' sense of 次第, but it attaches to the verb's 辞書形, is used in writing, and mostly describes unexpected actions by others that have already happened, with a second clause that cannot express volition or a request. 次第, by contrast, announces future actions, so the two are not interchangeable.
• たとたんN3Only the 'as soon as' sense is shared, and the two are not interchangeable. 次第 attaches to the ます形 with ます dropped or to an action noun and speaks of future arrangements, while たとたん attaches to the た形, describes things that have already happened, and takes a second clause that is usually an uncontrollable change and cannot express volition or a request.

次第だ／次第で expresses dependence on a factor; 〜次第では presents a possible outcome depending on that factor, while いかんでは／いかんによっては is more formal. や否や, なり, and たとたん describe an immediate occurrence, whereas verb-stem 次第 typically introduces a planned action to follow.