た途端(とたん)にThe instant or moment, As soon as, Just as

Both express actions occurring in close sequence, but differ in focus. たとたんに emphasizes the precise moment with suddenness, e.g. 'ドアを開けたとたんに猫が飛び出した' ('The moment I opened the door, the cat jumped out'). かと思ったら・かと思うと marks a quick change with surprise, e.g. '晴れたかと思ったら、すぐに雨が降り出した' ('Just when I thought it cleared up, it started raining again').

たところだ and たとたんに are similar in that both refer to something that has just happened, but with different usage and nuance. たところだ simply describes the timing, e.g. 家に帰ってきたところだ ('I just got home.'). In contrast, とたんに expresses that something else happened immediately at that moment. For example, 家に帰ってきたとたんに雨が降ってきた ('The instant I got home, it started to rain.').

直ちに and たとたんに both indicate swift timing, but differ in tone and structure. 直ちに is formal and emphasizes urgency, e.g. '直ちに避難してください' ('Please evacuate immediately'). たとたんに conveys something happening instantly after another event, often with surprise, e.g. '顔を見たとたんに泣き出した' ('She started crying the moment she saw his face').

Both refer to something that just happened, but differ in nuance. たばかり describes the state after completion, e.g. 帰ってきたばかり 'just got home'. とたんに means something happened the moment another did, e.g. 帰ってきたとたんに雨が降った ('The moment I got home, it started to rain.').

Both と同時に and たとたんに describe closely linked events, but they differ in focus. と同時に emphasizes simultaneity or coordination, while たとたんに conveys a sudden, often unexpected result following a completed action. Their grammar structure also differs: と同時に follows the non-past form, while たとたんに follows the past tense.

Both describe sequences where one action quickly follows another, but differ in tone and formality. 次第 is formal and indicates that an action will follow as soon as another is completed, e.g. '会議が終わり次第、報告します' ('I'll report as soon as the meeting ends'). たとたんに emphasizes sudden or unexpected results, often in conversation, e.g. 'ドアを開けたとたんに猫が飛び出した' ('The moment I opened the door, the cat jumped out').

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').