How does the tense and ある　in this sentence work


From my e-mail correspondence on italki:

「友達申請をしてくださって、ありがとうございます」は少し難しいかと思ったのですが、これから使う機会がある **と思うので**
、そう書きました。覚えてくれてうれしいです。

My attempt at translation: "Even though I thought that
「友達申請をしてくださって、ありがとうございます」 might be a bit hard, because **I think/thought**
that from there on...

A)...there is the chance you use it... B)...you have the opportunity to use
it...

...I wrote it that way. I'm glad you picked it up."

First: What's a bit irritating for me is that in this full sentence, there is
present tense all of a sudden in ...と思うので.... I know from my past questions
that japanese isn't too strict in the use of tenses, but I would like to know
why there is this sudden switch and what it expresses.

Second: I wonder wether I should use ある in the meaning of "existing" for
inanimate things, or in the meaning of "owning". Or are both meanings
basically the same in japanese? In my textbook, I once learned for ある that it
can express ownership (at least I think I did...^^) over the associated
object. But maybe this "extrasemantics" was just a means to make it easier for
the learner of japanese to understand the different functions ある appears in.

Third: In 覚えてくれてうれしいです I think she expresses that she's happy that I picked up
the grammar she had shown me. However, I just wanted to ask for confirmation
wether my understanding, which I tried to convey with my translation, is
correct or not, since the combination of "memorizing" and "giving a favor"
seems a bit strange to me. So I don't know wether I should trust my
understanding or not ^^




I feel like "Something something...たのですが、something something と思うので..." sounds
a bit incorrect. I would write "Something something...たのですが、something
something と思ったので..." and it sounds more proper. So I personally think it's
just her irregular way to write (but it will be still generally acceptable in
Japanese texting). And "ある" works as "To be" and "To exist" and refers to
things and objects that are non-creatures in English and it doesn't matter
what is physical or non-physical.

The phrase "覚えてくれてうれしい。" can be interpreted like this:
(あなたがsomethingを)覚えてくれた。だから、その事について私はうれしい(気持ちを感じている)。

I personally think even if the phrase is separated like this, the meaning and
the nuance won't be changed.




I feel like "Something something...たのですが、something something と思うので..." sounds
a bit incorrect. I would write "Something something...たのですが、something
something と思ったので..." and it sounds more proper. So I personally think it's
just her irregular way to write (but it will be still generally acceptable in
Japanese texting). And "ある" works as "To be" and "To exist" and refers to
things and objects that are non-creatures in English and it doesn't matter
what is physical or non-physical.

The phrase "覚えてくれてうれしい。" can be interpreted like this:
(あなたがsomethingを)覚えてくれた。だから、その事について私はうれしい(気持ちを感じている)。

I personally think even if the phrase is separated like this, the meaning and
the nuance won't be changed.

