How to use ふと correctly?

Question

I am struggling in understanding how to use ふと

ふと: suddenly, unexpectedly, unintentionally

Based on the brief explanation on this post,

ふと is used to express something unimportant that you don't think about, happens while you are unaware of it and thus it becomes unexpected.

This is an example with verbs that are "volitional"

その考えがふと思い浮かんだ That idea has crossed my mind unintentionally (it came to my mind without thinking about it)

ふと足を止めた。Suddenly, my feet stopped. (my feet stopped without thinking)

These are examples with verbs that are "non-volitonal"

ふと灯りが消える The lights suddenly turned off. (the lights turned off, I was unaware that it turned off)

Keeping this in mind, I tried making my own sentences but I don't understand why these are unnatural when I asked a native speaker.

ふとくしゃみをした I suddenly sneezed.

突然ゴキブリが現れてふと叫んだ。A cockroach suddenly appeared and I screamed unintentionally.

Why are these sentences unnatural, and is there an explanation of the logic behind what verbs can be used with ふと?

Answer 1

Monolingual dictionaries define ふと along the lines of "with no clear reason, indication or consciousness". The nuance of ふと is often closer to "somehow" rather than "suddenly". Your ゴキブリ example is unnatural because the cause for the scream is clearly the cockroach. You can say 思わず叫んだ instead.

Most usages of ふと are related to "subconscious" functions of the mind of a subject (typically first-person), for example ふと思い出した, ふと気になった, ふと興味がわいた, ふと我に返った, ふと目が覚めた. And ふと立ち止まった, ふと立ち上がった, ふと振り返った and so on are also fine (usually with a third-person subject) and it indicates someone vaguely noticed something is happening. However, くしゃみ is a physical reaction rather than an action of the mind, so it doesn't go well with ふと.