When do you use "נמצא" and when is it optional?
It is always optional. It translates roughly as "is located". If you are learning German you will find similar optional verbs of location such as "befinden sich".
Ha-ashpa nimtset ba-fakh ha-ashpa.
Colloquially: ba-pakh ha-ashpa.
So boring to do these first lessons again and again that I am too quick to get rid of it... And have to do it again. A nightmare. And i feel I am learning English (not my first language) and not Hebrew...
I have the same problem as English is also not my first language. Many mistakes are made because of this. But to me it is not a nightmare as i see that i am really progressing in Hebrew, finally!!
Can זבל be used instead of אשפה ?
אני חושבת שכן אפשר...
Why is it not בתוך?
Well, בתוך means "inside". That is possible, of course, but not as a translation of this sentence.
The garbage is inside the garbage bin. האשפה נמצאת בתוך פח האשפה.
As "Reverso" translates both words בפח and the word אשפה as garbage. Isn't that a double translation: garbage-garbage?
Not quite. פח is the container for the garbage, the bin, and אשפה is the garbage itself.