Verb in plain form + に: what does it mean?

Question

"感想を求められているのだと察した。"

"自分は何も言わず、頷くに留めた。"

"沈黙の感想を、彼はとても嬉しそうに受け止めた。"

As far I can see, the character understand an opinion he asked of him. So he doesnt' say anything and nods. The other person is happy to receive the impression.

But why is に used after a plain verb? And what grammar is this?

Answer 1

I can't comment on the grammar pattern in general or what verbs can come after に but in this case Xに留める means "limited to doing X".

自分は何も言わず、頷くに留めた。
I didn't say anything and limited (my response) to a nod of the head.

Answer 2

To add to @user3856370's answer. This is a specific case of a general pattern where in the cases of some verbs “〜に留める” can follow an entire sentence. These often have idiomatic usages that are slightly different from the verb, such as:

• “私は人間に決まっている -> “I'm obviously a human”
• “もう死んでいるに相違ない” -> “No doubt he's already dead.
• “冬はホットチョコレートに限る -> “Nothing beats hot chocolate in the winter”

Note that “〜だ” cannot be used in this pattern “〜だった” can, as in the first example, it must be omitted. “人間だに決まっている” would not be grammatical.

Another use of “〜に” after a verb is with some verbs of perception. This is essentially identical to “〜と” as in

• “私が思うに可愛い子。”　-> “He's a cutie in my opinion”
• “様子を見るに、多分生きていかない。” -> “He probably won't keep on living, looking at the situation.”

The final use is “〜には”, which is best seen as an inseparable part which can follow any verb. There's much written on this on the internet and it's long to explain but I'll provide some examples:

• “恋を呼ぶには気持ち悪い。” -> “It's disgusting to call this love.”
• “きれいでいるには努力が必要。” -> “It requires effort to stay pretty.”

Note that the second case can of course also get “〜は” behind it so “私が見るには” may not be the third case but the second, but the third case must always have “〜は” behind it.

Verbs cannot in general be followed by “〜に” outside of special circumstances like the above.