なりAs soon as, The moment, When

Both describe actions happening immediately after another. かと思ったら・かと思うと adds surprise or contrast, as in 雨が降ったかと思うと止んだ (No sooner had it started raining than it stopped). なり emphasizes immediacy without surprise, as in ドアを開けるなり走り出した (He opened the door and immediately ran out).

Both と同時に and なり follow the plain form of verbs and describe events occurring in close succession, but their nuance differs. と同時に means 'at the same time as' and emphasizes actual simultaneity. In contrast, なり highlights immediate consequence, something happening as soon as the first action occurs, and often implies a reflexive or automatic response.

Both express immediacy following another action, but differ in nuance and tone. たとたんに stresses suddenness or unexpectedness, sometimes with emotion, e.g. '立ち上がったとたんに倒れた' ('He collapsed the moment he stood up'). なり also marks instantaneousness, but feels more formal or literary, and implies a natural cause-effect link, e.g. '席に着くなり、食べ始めた' ('As soon as he sat down, he started eating').

Both なり and 次第 indicate immediacy but differ in tone and nuance. なり is a neutral expression suggesting a spontaneous or automatic reaction. For example, 布団に入るなり寝た ('He fell asleep as soon as he got in his futon'). 次第 has a more formal tone and is often used in business contexts to show that something happens immediately after a condition is met. For example, 終わり次第ご連絡します ('I'll contact you as soon as I'm finished').