よyou know; I'm telling you
よ hands the other person something they don't know yet or haven't noticed: a fact, a warning, an offer. English mostly does it with tone of voice.
telling
That train doesn't stop at the next station, just so you know.
How to form it
Sentence + よ
Plain or polite: 行くよ, 行きますよ, 寒いですよ. A noun or a な-adjective usually takes だ or です first: 休みだよ, 静かですよ.
Telling someone what they don't know
A fact they need, a tip, a warning, a correction.
The reception desk is on the second floor.
This cake is good.
A recommendation: they haven't tried it.
It's cold outside, just so you know.
Excuse me, you dropped this.
Does tomorrow's meeting start at two?
No, it starts at three.
Offering and reassuring
About something you will do for them, よ makes it an offer: leave it to me. To someone who is worried, it reassures.
I'll take you to the station.
You don't need to worry; it'll be fine.
ね, よ or よね?
The curry at this restaurant is spicy, isn't it?
You are both eating it.
The curry at this restaurant is spicy, so be aware.
They haven't tried it. You are telling them.
The curry at this restaurant is spicy, right?
You have both eaten here before.
ね: they can see it or know it too. よ: they don't know it yet. よね: what you think or remember, checked with them.
So when they can see or feel it too, よ has nothing to hand over.
Sounds odd: 暑いですよ。
It's hot, I tell you.
Odd as a reply to 今日は暑いですね: it tells them what they have just told you. The reply is そうですね.
The same split decides between two everyday answers.
That sounds good.
A friend says 映画を見ませんか. You like the idea too.
Sure, go ahead.
A friend says このペン、借りてもいいですか. They asked, so you give them the answer.
When it pushes
よ makes you the one who knows. In a friendly voice that is helpful. Said sharply, or about something they know perfectly well, it pushes back.
It's already eight.
It isn't needed every time you tell someone something. A plain sentence tells too, and よ on every one sounds like a lecture.
On a request it presses, where ね is gentle. After 行こう or 行きましょう it coaxes.
Hey, wait a minute!
They aren't waiting, or you don't like what they just said.
Come on, let's go together.
Close neighbours
ねN5
Turns to them and asks them to agree, confirm or say okay.
This park is quiet in the morning, isn't it?
Check yourself
1A stranger is leaving the café, and their umbrella is still by the chair. Which do you say?
Choose an answer to see why.
2A classmate asks you this. Which answer means "Sure"?
ここに
Choose an answer to see why.
3You think the test is on Friday, but you want to make sure. Which fits?
テストは
Choose an answer to see why.
More detail
ぞ and ぜ do the same job more roughly: mostly men's speech, and commoner in anime than in real life.
Here we go.
Right, I'm going to give it my best.
Said to yourself. Anyone says this ぞ.
Without だ, きれいよ is traditionally women's speech: today mostly older women, and women in fiction.
More sentences
The shop is closed tomorrow, just so you know.
Mr. Tanaka has already gone home.
That art museum is closed on Mondays, just so you know.
Tomorrow's meeting is at ten, just so you know.
It's already seven, you know.
This route is shorter, right?