In theory, the project should be completed in a month, but in practice it often takes longer due to unexpected issues.

in 是介词，表示"在...之内"或"在...方面"。 theory 来自希腊语 theoria，意思是"观察、思考"，指通过系统思考形成的知识体系或原理。 in theory 是一个固定搭配，表示"从理论上说"或"按道理讲"，强调与实际情况(in practice)相对的理论层面。