In theory, the journey ought to take three hours, but in practice it usually takes four because of roadworks.

in 是介词，表示"在...方面"或"在...范围内"。 theory 来自希腊语 theoria，意思是"观察、思考"，在现代英语中指"理论、学说"。 in theory 是一个固定搭配，表示"从理论上说"，与实际情况相对。