i am making a mandarin course recently.
了 着 过 have been put into the same group.
they are all used following a verb.
"verb+了" is similar with "done" in english.
"verb+着" is similar with "doing"...
"verb+过" is similar with "have done "...
there is a close relationship betweem the word(了,着or过) and the verb.they constitute the whole.
but 正在 is different.
it just a general adjective to describe the state of the moving compared to 着.
another 2 words are put in the same group with 正在 in my course.
已经, means a movement already done
还没, means it's a negative. the movement didn't finish
正在, the movement happening
hope it helps