"The fish was slowly swimming in the river."
When you want to use gerund, you have to add imasu (います) in the end of the verb. But to add it you must change the verb to the DE (で) form. oyoide (およいで) is the DE form of the oyogi (およぎます) verb.
で is an event marker. Although keep in mind で does other things too. For example, 鉛筆で書きました (えんぴつでかきました) means I wrote it with a pencil or by means of a pencil. So the same particle can do entirely different things.
Why is it not およぎました? Would it be "the fish swam" then ? I don't understand how the verb is conjugated, can someone explain ? Thank you very much !
It's a different case of the verb.
およぎました = swam (So yes, it would be "the fish swam" if this was used like you said!)
およいでいました = was swimming. It's a combination of およいで, the で form of およぐ (to swim) and いました, the past tense of います, to exist.
The way I think of it is "In the past, the act of swimming existed" therefore, it becomes "was swimming".
GreggoGato has a good breakdown a few comments below as well.
(P.S. if this is wrong at all, please comment and explain why it is so I can come back and correct it so people can see the correct version. I'm still learning, too)
That's the difference between swimming late (i.e. at a late time rather than while late for something which would be おくれて) and swimming slowly.
I just wonder why they don't like 川で魚がゆっくりで泳いでいました。Too colloquial?
I think both of them are accepted. But が is only used to indicate the subject.
さかな (fish) が 川で <--- In the river ゆっくり <--- slowly およいでいました。 <--- Was swimming
およぐ ＜－－ Swim
およいでいます <-- Is swimming (conjugated continuous form of およぐ)
およいでいました <-- Is swimming (conjugated past continous form of およぐ)
This is like the "ing" form in English - "The fish was slowly swimmING in the river". It means that the action happened over an indefinite period of time, not just at one moment.
Its actually て + いました form. The て + います form indicates the "ing" portion (泳いでいます - swimming), but you turn the ます into ました (泳いでいました - was swimming).
So for other examples it has been に in reference to fish swimming, location - ni. This has been described because it's more a natural state of them being, not a specific action as used with で in other examples. But because the fish is swimming slowly, I think this is no longer the natural state and now becomes a different action, which gives the justification for で. Please correct me if I'm wrong, I'm still working out the difference between these particles.