ふと is an adverb meaning "suddenly" or "happen to", for something that happens in your own head or senses without your meaning it to: a thought comes, you glance up, you notice. It goes before the verb or at the start of the clause.
、。
I happened to look out of the window, and it was snowing.
。
I suddenly remembered something from long ago.
。
I woke up for no reason in the middle of the night.
The verbs are mostly ones of thinking, noticing and looking: , , , , .
、きごう？
It just occurred to me – isn't tomorrow a day off?
、。
Before I knew it, it was already ten at night.
ふと or 突然
is "suddenly" for anything abrupt, including events in the outside world. ふと is only for what goes on in the person: no intention, no effort, no warning. An earthquake or a phone call can't happen ふと.
。
Suddenly there was an earthquake.
。
A good idea came to me out of nowhere during the meeting.
Lingaku