yah they could mean something different, and it can be tricky in 古文 (classical Chinese)
他只是在圖書館看書。 He just studies in the library.
他只在圖書館看書。 He studies only in the library.
2. 學、學習：學習 could just mean learning, then it is a noun; 學 could be followed by object
學習是快樂的。 Learning is delightful.
學跳舞是快樂的。 Learning dancing is fun.
你住（第）幾樓？ What floor do you live?
你住第幾樓層？ (...well it's understandable)
"幾個" can be a question already, = how many
I could only think of some examples~