までだ・
The verb in front decides the meaning. After a dictionary form it is calm resolve: if it comes to that, "I'll simply do this". After a た-form it plays down something done: "I only did this", and no more should be read into it.
I'll simply
If the trains stop running, I will simply walk home from the station.
I'll simply
Verb (dictionary form) + までだ／までのことだ
I only
Verb (た-form) + までだ／までのことだ
までのことだ is the fuller way to say either: "that is all there is to it". Polite までです, までのことです.
One verb, two meanings
I'll simply do as I'm told.
Still to come. Your mind is made up.
I only did as I was told.
Already done. There was nothing more to it.
Two look-alikes are other expressions: …ばそれまでだ, "then that's the end of it", and …までもない, "no need to".
I'll simply
A condition usually comes first, with なら, たら or なければ. It is mostly your own move, or your side's.
If no one else will take it on, I'll simply take charge myself.
If the plan is not approved, we will simply revise it and submit it again.
If talking can't settle it, we will simply leave the decision to a third party.
I only
The reason usually comes with it, often after a denial: not for praise, not to object, just this. It can be modest or cool.
He was in trouble, so I lent a hand. That's all.
I wasn't opposing the rule. There were some unclear points, so I merely asked for clarification.
I only did what anyone would have done.
A set reply when you are thanked or praised.
Close neighbours
だけだN5
What people say for both: 歩いて帰るだけだ (or 歩いて帰ればいい), 答えただけだ. までだ is the firmer and more deliberate.
I was asked, so I just answered.
しかないN3
Every other option is gone, and you are cornered: "I'll just have to". With までだ you have a move left, and you say so calmly.
The last train has already left, so we have no choice but to take a taxi home.
"Then that's the end of it": everything done so far comes to nothing.
In this negotiation, if we lose the other side's trust, it is all over.
までもないN1
"No need to go that far": a step is not needed, because the answer is already plain or the matter is too small.
For a wound this minor, there is no need to go to the hospital.
After a verb, plain まで is "until", and までです can simply say when something ends.
This sale runs until stock is gone.
Check yourself
1What is this person saying?
私は
Choose an answer to see why.
2Which one fits?
この
Choose an answer to see why.
More sentences
If the other party does not honor the contract, the company will simply take legal action.
If the data cannot be recovered, we will simply start the investigation over from the beginning.
Whoever we are up against, we will simply play our kind of baseball.
The kind of thing players say before a big game.
They seemed lost, so I simply showed them the way to the station.
I simply kept practicing every day because I wanted to improve. There was no special secret.
I wasn't looking for praise. I took on that role simply because I thought it was necessary.
I merely pointed out the problem at the meeting because I felt there was a risk.