What does it mean to use a する verb without する?

Question

その絵はほとんど完成だ。

I find this sentence quite confusing. 完成 means the act of completion, so I don't understand why だ is used instead of a variation of する. The meaning seems different than a sentence like 今は勉強だ where it says that something is in the process of the する verb rather than being in the state after the する verb as implied in the above example.

Also, when a character says 了解 after receiving a command, are they omitting した or です? It's very complex since there are times when a character will say "これを召喚！" and it means "I will summon this" or "I summon this" and despite following the format of omitting something, they mean different things.

Answer 1

Q. What does it mean to use a する verb without する?

A. 完成 is a noun (meaning 'complete' or 'completion'). する is added to it to make it a verb (meaning 'to complete'). For example "That picture is mostly complete (in a state of completion)." (その絵はほとんど完成だ。）as opposed to "I've mostly completed that picture." (その絵はほとんど完成しました。）

When you see 熟語{じゅくご} (compound characters, such as 完成、勉強、了解、召喚 etc.) followed by する it is taking the noun form and making it a verb. As for looking for what is being omitted, you can look at the context and particles surrounding the 熟語. For example, when someone says 了解, they could be saying two different things with the basically the same meaning. 了解（しました）-I understood/got it/roger that or 了解（です）-lit. (It) is understanding/comprehension (i.e., I understand).

Answer 2

Basically する verbs are a combination of する with a noun that has an inherent meaning of an action/state - almost always a Chinese compound or single character. The する is really nothing more than the "Japanese agent" of that Chinese noun - taking care of the tenses, endings, etc. that the the kanji itself can't express.

Since the kanji part already has the basic meaning necessary for understanding what its about, and has indeed come (at least in spirit) from Chinese where it can function as a verb on its own, it often happens that the する part is simply left out if not considered necessary. The noun on its own is already understood as a verb.

Like する, だ here is a vague element serving to complete the sentence. It's not equating "painting = near completion", but saying, "The painting? Almost finished", like you would say in very casual English.

The idea is that in this sentence, it's not important to the speaker to stress the completion as a finite verb (one with a tense etc.), only to generally say that its nearing completion.

Regarding 了解, as far as I know it's short for 了解しました or 了解いたしました.

Answer 3

Without する the compound becomes a noun. Let's compare:

その絵は完成だ。 That picture is complete.

その絵は完成した。 That picture is completed OR I completed that picture.

Another quick example:

明日は遠足だ It's our school trip tomorrow 明日は遠足する tomorrow we will go on (= do) our school trip.