Questions & Answers about Ґудзик лежить біля нитки.
Because біля requires the genitive case.
The base form is нитка (thread). After біля (near, by), it changes to нитки, which is the genitive singular form.
So:
- нитка = thread
- біля нитки = near the thread
This is a very common pattern in Ukrainian:
- біля столу = near the table
- біля машини = near the car
- біля нитки = near the thread
Ґудзик is in the nominative singular because it is the subject of the sentence — the thing that is doing the action or being described.
In this sentence:
- ґудзик = the button
- лежить = lies / is lying
- біля нитки = near the thread
So ґудзик is the thing that lies somewhere.