～てしまうTo do something by accident, To finish completely・totally, Unfortunately

はじめる attaches to verb stems to mean 'to begin' or 'start' doing something, e.g. 食べはじめる 'to start eating'. In contrast, てしまう・ちゃう follows the て-form and can express doing something unintentionally or completely, e.g. 食べてしまった・食べちゃった ('I ate it unintentionally.' or 'I ate it all.').

てしまう・ちゃう and 切れない can express opposite meanings. One nuance of てしまう・ちゃう is to do something completely, e.g. 食べてしまった has one potential meaning of 'I ate it all.'. In contrast, 食べ切れなかった would mean 'I couldn't eat all of it.'. Note that てしまう・ちゃう can also express regret or an unintentional action, which 切れない does not. For example, 食べてしまった could also mean 'I ate it (even though I didn't intend to).'.

てすみません is used to apologize for doing something, e.g. 遅れてすみません ('Sorry for being late.'). It often pairs with てしまう・ちゃう, which adds a nuance of regret or accident. For example, 遅れてしまってすみません・遅れちゃってすみません ('I'm sorry I was late.'), which implies that you didn't mean to do so and feel bad it happened.

おわる attaches to verb stems to mean 'to finish doing (A)', e.g. 食べおわった ('I finished eating.'). In contrast, てしまう・ちゃう uses the て-form and can also mean doing something completely, but often adds a nuance of regret, finality, or lack of intention, e.g. 食べてしまった・食べちゃった ('I ate it and now it's gone.' or 'I didn't mean to eat it.').

切る conveys the idea of doing something completely, which overlaps with one nuance of てしまう・ちゃう, but their usage differs. てしまう・ちゃう can imply that the action was unintentional or against one's will, which 切る cannot. In other words, 食べ切った simply means 'I ate all of it.', while 食べてしまった・食べちゃった also can have the meaning of mean 'I ended up eating it (even though I didn't intend to).', which 食べ切った cannot.

上がる・上げる can express the idea of completing an action, which overlaps with てしまう・ちゃう, but they differ in nuance. てしまう・ちゃう can express regret or lack of control, while 上がる・上げる focuses on the act of producing a finished result. For example, 書き上げる 'finish writing'. In contrast, てしまう・ちゃう can be used with any verb, regardless of whether a tangible result is produced.

つい and てしまう・ちゃう can both express doing something unintentionally or against one's better judgment. つい appears at the beginning of a sentence or phrase, while てしまう・ちゃう is a verb ending. They are often used together to emphasize the nuance of regret or unintentionally, e.g. つい食べてしまった・つい食べちゃった would imply 'I ended up eating it (even though I didn't mean to).'.