Do you know about the て form of verbs? For verbs where dictionary form ends in む, ぶ, or ぐ, the て form uses で instead of て.
So maybe you've seen て forms like this
たべる > たべて
きく > きいて
ある > あって
and so on
For those like I was mentioning
よむ > よんで
とぶ > とんで
およぐ > およいで
So, I wouldn't try to think about this で as being a separate thing from the verb 泳ぐ (even though it is possible to think about て forms that way, but for beginners just imagining it as one way to conjugate is okay).
およいでいます means "is swimming." It's the present continuous form. て＋いる makes the "is verb-ing" grammar.
If you still have questions let me know. I wasn't quite sure what inspired the two numbered questions you asked, so sorry for not really addressing those specifically.
Thank you so much for clearing that up. I was not even thinking that it could be a -te form since the sound was changed to -de. That helps a lot.
The app I am using really does not explain their lessons, so I have to try to pick up the grammar rules from other sources.
Use of で in this sentence
I was given the following sentence on a learning app:
男の子は池で泳いでいます。
A boy is swimming in the pond.
I am confused about two points:
1. They said 泳いで is pronounced "oyoide" and not "oyogi de" as I would expect.
2. Other sites said 泳いで means "by swimming," but that is not how it is being used in the sentence.
Can anyone please explain?