When is it natural to use で and に and when not to use them

Question

In my previous question I wanted to know how to say

If we continue at this rate I will be fluent by Christmas.

Both answers were really helpful and I now know how to say it but there is still something that bothers me:

All suggestions were of the form

〜、クリスマスまでに話せるようになるわ

where 〜 is the word for "at this rate" like for example このペースだと、この調子なら、このままいけば.

As a non-Japanese speaker I have the strong urge to make a construction like this:

〜で続けば、クリスマスまでに話せるようになるわ

For one, the urge to use "to continue" and for another the で particle (or perhaps に).

So I was wondering, how unnatural on a scale of 1 to 10 is this? I'm trying to understand why I should probably not use either に or で here. Please could someone explain to me exactly when it will not be a good idea to make a construction like this?

For the record: I do know that に may be a particle indicating location like e.g. on the desk = 机の上に or "by" as in 本田さんに言われた and many more such "obvious" uses. But what I am asking here in this question seems way more subtle to me (beyond my current language feeling of Japanese).

Answer 1

I'm not a native speaker so it might not be the most natural way to say it but I would say :

このままではクリスマスになったらペラペラだよ

I don't understand why you want to use 【で】 or 【に】 here, the only 【に】 I would use is with 【～になる】. You should explain why you would use these particles and when, make some examples, it would be easier for us to see where you're stuck at.

Answer 2

I would argue there's no real difference between English and Japanese here. In English you can very well say "At this rate, xxx", where it is understood that something is continuing at the same pace or in the same way. Likewise, in Japanese

このままいけば、クリスマスが来たらもうペラペラになってるよ

However, if you need to specify what the verb is, then it is common to say things like

このまま勉強し続ければ...
このまま付き合い続ければ...

etc

Answer 3

I'm not a native speaker but it feels natural to say it like this:

このように続けばクリスマスまでにペラペラになるだろ

Using このように gives more the impression of how someone is doing a verb. Whereas で is used for something being or existing in a state.

Answer 4

Maybe a technical point, but I would say 続けば is not quite correct in this case.

Using a form of 続く has an implication of an external event which is continuing regardless of you, rather than a volitional action by you. Since it is you who is studying, your actions are volitional and therefore would be better expressed by using the provisional 続ければ or the conditional 続けたら.

As for に and で、 try to think of までに as a set phrase for having something completed by a certain time.