すっかり shows that the old state is gone

すっかり is an adverb meaning “completely,” “entirely,” or “all.” In everyday stories, it often presents a change or result as complete: the earlier condition has disappeared, nothing relevant remains unfinished, or something has fully become a new state.

That is why すっかり sounds natural with verbs such as 変わる, “change”; なくなる, “disappear”; 片づく, “get put in order”; 忘れる, “forget”; and 良くなる, “get better.” It also fits a new resulting state, as in すっかり元気になる, “recover fully and become well again.” Kotobank defines the word through having nothing left and having completely reached a state. NINJAL teaching material uses it for recovering from a cold, forgetting homework, becoming an adult, and finding that a town has changed.

The tense tells you where the speaker stands. すっかり変わった looks back at a completed change. すっかり変わっている describes the resulting state that the speaker now sees. すっかり忘れていた often appears when someone has just remembered an obligation that had been entirely absent from mind. The word itself does not say whether the result is good or bad. Recovery can be welcome; forgetting an appointment can be a problem.

Three nearby words answer different questions

全部 means “all” or “the whole thing.” It points to a countable set or the full extent of one thing: all the menu items, the whole song, every page, or all the money. The Japan Forum contrasts this with すっかり directly. 全部歌う means sing the whole song, while a change such as すっかり春になった presents the season's complete transition. The boundary can overlap when a set is treated as one changed whole. For example, both すっかり and 全部 can describe forgetting all the words one learned. Ask what matters: the complete before-to-after change, or every member of a set.

完全に also means “completely.” The adjective 完全 describes something with no missing part, deficiency, or unmet condition. As an adverb, 完全に can evaluate whether understanding, separation, mastery, or another condition is complete. It overlaps with すっかり in some sentences. A useful tendency is that すっかり feels at home in a personal account of how an earlier state changed, while 完全に can sound more like an assessment of completeness. This is not a rule that makes the other choice impossible.

さっぱり is not a general replacement for すっかり in modern everyday use. It can describe a clean, refreshed, or light feeling, as after a shower or in さっぱりした味, “a light, refreshing taste.” With a negative expression, さっぱり分からない means “not understand at all.” Those meanings are different from simply reporting that a shop, person, or situation has completely changed.

Returning to a café after the renovation

Mika and Jun meet at a neighborhood café that was closed for renovations. Jun had also missed work with a cold, so Mika checks on him before they go inside.

The earlier condition was being sick. 治って and 元気になった present the completed recovery and its welcome result. もうすっかり is a natural combination when “already” matters: Jun is reassuring Mika that the illness no longer describes his present condition.

They open the door. The counter has moved, the tables are different, and the wall that divided the room is gone.

変わってた is the conversational form of 変わっていた. Mika is not counting individual alterations. She sees one earlier interior and one new interior, then reports a complete before-to-after change. This matches NINJAL's conversational example about returning after several years to find a town transformed.

Mika tries to remember where their usual table stood, but the old picture in her mind is faint.

Here the missing earlier state is a memory. 忘れてた is the spoken form of 忘れていた. Mika recognizes the lapse only when the renovated room gives her a reason to compare past and present. This is a common shape for an overlooked appointment, task, name, or detail: すっかり忘れてた.

The new counter has several unfamiliar drinks and desserts. Jun wants to consider every item before ordering.

全部 is useful because the menu offers a set of choices and Jun wants to see every one. The sentence is about coverage, not about an old menu changing into a new state. This same logic works with 全部読む, read all of it; 全部食べる, eat all of it; and 全部でいくら, how much in total.

They choose their drinks, but the café now takes orders through a phone app. Mika opens it and cannot tell where to begin.

With 分からない, さっぱり strengthens the negative: Mika has no understanding of the procedure. すっかり分からない is not the everyday pattern to copy here. If she had understood before but had now forgotten the procedure, she could instead say 使い方をすっかり忘れた, “I completely forgot how to use it.”

Jun finds the first screen but realizes that genuine mastery will take more than one visit.

完全に evaluates mastery against a full standard: knowing all the functions well enough to use them freely. The sentence is not narrating an old state that has vanished. A staff member shows Mika and Jun the two steps they need today, they place the order, and they leave full mastery for another visit.

When choosing, identify the question your sentence answers. Did an earlier condition disappear or fully become a new result? すっかり is a strong candidate. Are you covering every item in a set or one whole object? Choose 全部. Are you judging whether a standard has been met without anything missing? 完全に may fit. Are you describing a refreshed feeling, light taste, or no understanding with a negative? That is common territory for さっぱり.