の・
To say you forgot to do something, or saw it happen, the action has to become a noun first. の or こと after a plain verb does that: 読むの and 読むこと are "reading".
I forgot to do my homework.
I heard my father singing.
My hobby is walking around unfamiliar towns.
How to form it
Plain-form verb + の ／ こと
Any plain form: 行く, 行かない, 行った, 行っている. It stays plain in a polite sentence.
Making a verb into a noun
English says "I forgot to take it". In Japanese what you forget takes を, and a verb can't stand in front of を by itself. It takes の first.
Not said: 薬を
(I forgot to take my medicine.)
A verb straight before を.
I forgot to take my medicine.
の makes 薬を飲む one noun, and を marks it.
Nothing inside changes. 薬を keeps its を, and the one who does it takes が, not は.
I am waiting at the entrance for my friend to come.
友達が来る + の: "my friend coming".
の or こと?
In most sentences either one works. の is what people say in conversation. こと is a little stiffer and suits writing.
I like traveling alone.
In an essay, or introducing yourself to a room.
Only の: it happens in front of you
What you see or hear happening takes の. So does what you wait for, help with or stop.
A child can be seen running in the park.
My mother helped me cook.
I stopped the dog from entering the garden.
Only こと: before です
To say what a hobby, a dream or a job is, it is こと before です.
My dream is to work in Japan.
Not said: 私の
(My dream is to work in Japan.)
A verb + のです is something else: it explains a situation.
The time inside
The verb before の has its own time.
I forgot to turn off the light.
消す: it didn't get done.
I forgot that I had turned off the light.
消した: it was done, and that slipped your mind.
こと is also a word for "thing"
こと is an ordinary noun too: a thing you can't touch, something said or done. With a description in front, it is "what …".
I wrote down what I heard yesterday in my notebook.
Please do not forget what you promised.
Set patterns built on these
Liking doing something, or being good or bad at it.
My younger sister likes singing.
のは〜だN5
Holds the key detail back for the end: "the one who came was Tanaka".
The one who came yesterday was Tanaka.
Close neighbours
のN5
After a verb, の can also stand for a thing you both know about: "the one I bought".
Please show me the one you bought yesterday.
んです・のですN5
At the end of a sentence, のです, usually said んです, explains a situation.
I'm going back to my country tomorrow, you see.
Check yourself
1"I forgot to buy milk."
牛乳を
Choose an answer to see why.
2Which one fits?
私の
Choose an answer to see why.
3Which one fits?
と
Choose an answer to see why.
More detail
Deciding and promising usually take こと.
The company decided to continue remote work.
More sentences
From the window, snow can be seen falling.
Walking every day is important for your health.
I became accustomed to speaking in front of people.
The committee decided to raise the membership fee starting next year.