Japanese: Simple sentence chaining; て-form (polite and casual), でvsて, and past tense

[Call-Me-Captain]

Hi!

I think it's quite cool how much one can learn by trying to translate one simple sentence.
Recently, I came across the following sentence: "I knew you were an angel" (At least that's what it amounts to in English.)
Since I knew the only word required to translate it, I tried my luck.

At first, I got the following:
(私は)知まして、君が天使です。
While it is wrong, I do not know all the reasons as to why.
(GoogleTranslate translates this back to "I know you are an angel" - So the present form of what I want to achieve. However, the present form has a whole different connotation to it than the past one. At least in English.)
I basically thought that I could chain together these two sentences: "I knew." and "You are an angel."
While the て-form might not have been the best choice here, as I am unsure as to whether it implies the same that the word "that" does (Not sure how to call this in English. Causation? Probably not, as neither of the two sentences are a consequence of each other...), it did raise a few interesting questions for me.
First of all, about the order of the sentences: Imagine a complex compound sentence. Which sub-sentences will go first?
In this case, it's obviously a bit simper as there are only two sentences to be chained together.
After thinking about it, I remembered that the "actual" verb always has to go at the end of the sentence.
Therefore, assuming nothing else was wrong, the above should become:
君が天使でして、(私は)知ました。
However, it still looks wrong (Even though I really don't have a lot of experience with the language), and GoogleTranslate translates this back into "You were an angel and I learned" - Which is completely different.
Another, this time very specific, question this raised, was whether です was actually the right verb to use for the "are" part. And if it is, how does the sentence "sound" with it? What "level of politeness" is it, and in what context could it be used?
For the second part, I used the verb 知る instead of 分かる because after reading a post comparing the two I felt like the first one would be more fitting. I tried to use the polite ます form, mainly because it feels easier to use.
If I were to make it more casual, would the following be correct (Not correct as a whole, but correct in the sense that it does not introduce any other mistakes that weren't already previously there.)
君が天使だって、(私は)知た。
Here's the weird thing: GoogleTranslate actually translates this one back into what I wanted.
Is it really right, though?
And if it is, or kind of is, what does it sound like in Japanese? How does it, I guess, feel?

Now, I did get some help from another source, and the person who helped me wrote that the following could be used:
君が天使だって知っていた。

Here, the first part is the same as what I got previously, so the casual form of "You are an angel.". However, the second part is very different.
With the help of Tae Kim's Grammar Guide, it seems like that form would be similar to an "-ing" form in English: So something that happened but didn't stop happening yet. I can kind of see how that would apply here, although I have to say that I am not 100% sure.

Finally, here is what GoogleTranslate gives me when I input the sentence in English:
あなたが天使であることを知っていました.
Up to angel, there is no difference, obviously (Besides 君/あなた, which is just a small difference.)
However, I do not understand the である part. I read that the こと part converts the previous verb into a noun?
The last part seems to be similar with what I had above, just in the polite form.
But yeah, I don't quite understand how the sentences have been linked.

So, considering this one sentence in English, what would the best possible different translations be in Japanese? How would the "feel" change based on the translation? What is wrong/correct about the translations above?

Finally, one last question I had about the て-form was that I sometimes saw sentences using て and sometimes で, and I'm not sure as to why.

Help would be greatly appreciated. Thank you very much!

Sincerely
CMC

[vonPeterhof]

I believe the simplest translation would be 君が天使だと知っていた (for a polite version just replace 知っていた with 知っていました; you generally only need to mark the main verb of the sentence for politeness, so だ would remain unchanged). There is a limited number of ways to link two clauses where one is a variation of the verb 知る and and the other is a fact that is known. と is probably the most common one. ことを is another, and slightly more formal one (and である is simply the formal non-polite equivalent of だ/です). The て forms aren't used like that. The thing about だって is that it's not actually the て form of だ (that would be で); you got the right translation due to a happy coincidence. In your sentence and the sentence suggested by your source だって is the copula だ plus the particle って, which in this case is a casual equivalent of と.

Now as for the tense of 知っていた, that might just be something better memorized as a quirk of the verb 知る: when you're using it in the sense of "to know" (as opposed to "to learn/find out") you're supposed to use the "continuous" form in the affirmative and the simple form in the negative (so the negative equivalent to your sentence would be 君が天使だと知らなかった; the polite versions would be 知りませんでした in the written/more formal language and 知らなかったです in the spoken/less formal language). You can look here if you want a detailed explanation of why it works like that. Also for the record, if you do need to use the "simple past" form of 知る, it's 知った, not 知た (the polite version is 知りました).

As for the verbs whose て-forms actually have で instead, that has to do with historical sound changes in specific circumstances. In Classical Japanese all て-forms had the pattern -ite (so 知って would have been 知りて), but for modern Japanese you just need to memorize which verb endings produce which て-forms.