I may be wrong but here is what I think...
"Watashi wa benkyō ni ōku no jikan o tsuiyashimashita."
I _ Studying_(a lot of time) spent ( past verb )
ooku is not to be confused with oku=put
But it still is easy to remember, oku ( put ) and benkyou(study) ni ( particle is equivalent to "in")
So "oku ni benkyou. is put in time" but now we have to use Japanese grammar and the Ooku instead...
We now have in "studying put in time"
Ooku = many/much.
So it makes sense that putting in/Much time is confusingly similar-sounding.
So with that aside we have-
"I spent a lot of time / much time studying Japanese."
Spent is past form. Mashita is the correct ending to mean something in the past.
Ni oku no jikan
jikan ( Time ) No is possessive
so "In studying's time, much."
Recap Dictionary defined reasoning translations are:
Eng: "I in studying's time, much spent"
Jap: watashi ha benkyou ni Ooku no jikan wo tsuiyashimashita."
I think if you're a native English speaker it's easy to over-complicate the sentences, I do it all the time but I feel this helps me truly understand the uniqueness of Japanese. Sometimes you need to remember in Japanese to refer to the past you say mashita with past verbs and you don't need to always add what I call in English "Unnecessary words" when you're just learning the language(for example). I recommended learning words like "lately" when you're fluent. It's less confusing. At least for me because in Japanese we don't have the same phrases. There are different ways of saying lately. From what I looked up.. Hope this helps.
Ja Matta ne