のだから・
The reason is something the other person already knows, and you use it to press what ought to follow: advice, a request, a judgment. In conversation it is んだから.
You're not a child any more, so think it through for yourself.
You made a promise, so please see it through to the end.
How to form it
Plain form + のだから／んだから
A noun or a な-adjective takes な: 学生なんだから, 静かなんだから. Polite: んですから.
- Before it
- A fact the other person knows or can't deny.
- After it
- What ought to follow from it: advice, a request, an order, "should", "let's", "of course".
- Tone
- A push. Warm when you are encouraging someone, sharp when you are telling them off.
- Said to
- Friends, family, people below you, yourself. To a boss or a teacher it easily sounds like a lecture.
In use
It's your first time, so you don't need to be afraid of getting it wrong.
We've come all this way, so let's look around a bit longer.
With せっかく: having gone to the trouble, let's make it count.
The deadline is tomorrow, so let's get it checked today.
から or んだから?
You have an exam tomorrow, so go to bed early.
A reason and an order.
You have an exam tomorrow, you know, so go to bed early.
The same, with "and you know it" added.
Because it leans on what the other person knows, it can't simply report why something happened. To someone who didn't know, it sounds like arguing. For a plain reason, people say から or ので.
Not said: 熱が
(I had a fever, so I stayed home from school.)
熱があったので、学校を休みました.
Stopping at んだから
People often stop there, to insist or to complain, and leave the rest unsaid.
Honestly, I was worried about you, you know.
Close neighbours
からN5
Plain "because". The reason can be news to the other person.
I'm not going today. Because I have no money.
のでN5
The soft "because", for explaining yourself to a boss or a teacher.
I'm not feeling well, so I'm going to head home early today.
ものだからN3
The opposite attitude: a reason the other person didn't know, given in the hope that they will understand. The "because" of excuses.
The traffic was terrible, so I ended up being late for the meeting.
Check yourself
1Your friend says this to you. What is she doing?
日本に
Choose an answer to see why.
2Which one fits?
もう
Choose an answer to see why.
More sentences
You're our guest, so please, stay seated.
The polite form. A push for the other person's own good is welcome from anyone.
You're not alone, so come and talk to me when you're in trouble.
This method is the safest, so there is no need to change the procedure.
Now that enough data has been collected, there is no reason to put off the conclusion.