Questions & Answers about Þegar pelinn er tómur, legg ég hann í vaskinn.
Þegar means when here. It introduces a subordinate time clause:
Þegar pelinn er tómur = When the bottle is empty
That clause sets the time or condition for the main clause:
legg ég hann í vaskinn = I put it in the sink
So the whole sentence has the structure:
When X happens, I do Y.
Not usually. Peli most often means a feeding bottle / baby bottle rather than just any bottle in general.
So if you see pelinn, the image is often more like a baby’s bottle than, say, a soda bottle or wine bottle.
A more general word for bottle is often flaska.