I'm assuming "instoppen" is a separable verb here. And separable verbs are something that I don't think I will EVER fully understand. But would "zet" be incorrect for this sentence? I'm resorting to guessing for a lot of these exercises, and I don't feel like I'm actually learning anything.
No, there is no separable verb in this sentence.
"Stoppen" is the verb used for putting things inside of other things*. After putting it there, you would use the verb "zitten" to describe its location.
- Ik stop de groente in de mand. De groente zit in de mand.
"Zetten" is used for placing things in an upright position. After that, the object is "standing" in its location.
- Ik zet de doos op de tafel. De doos staat op de tafel.
Finally, "leggen" is used to place things in a prone or flat position.
- Ik leg het boek op de tafel. Het boek ligt op de tafel.
For more examples, I recommend the Tips & Notes for this skill (available on the website, if you're using the app). You have to develop a feeling for these words, because there are few hard and fast rules. Which word you use really depends on the meaning of the sentence, rather than the grammatical structure.
*for this purpose, you might also encounter "doen" instead of "stoppen": Ik doe de groente in de mand.
Great examples. Thanks. It would be good if you could put them with the table that's in the Tips & Notes for this skill. That was all the notes needed to complete them!