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


In my [previous
question](https://japanese.stackexchange.com/questions/25683/how-to-say-at-
this-rate) 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).




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.

