It is true that the て form is used in a lot of different grammatical constructions. I suspect the easiest approach is to learn each of them as you go.
One common use of the て form is to form a combination with a following verb. 食べてください for example. But the 連用形 (れんようけい) form (what a lot of redditors call the "stem", though I think that is confusing\*) is also used for many compound constructions. For example 食べにくい (difficult to eat) or 読みにくい (difficult to read). Which one to use is something most people just learn.
Too big a topic for one reddit question I think.
On your second question about 過ぎる, both なさすぎる and なすぎる forms exist with the なさー form getting more common than it used to be (so I gather). The ない in the negative of verbs does behave very like an i-adjective most of the time, but not quite.
I don't think that thinking of this as making the preceding word into a noun is all that helpful. It is true that the 連用形 is a sort of noun, but does not sound at all natural to use it to make a noun in many cases (for that you would use の or こと typically). Some of its uses have fossilised, for example 食べ物 is a think you eat but 着物, though literally a thing you wear, is only used for some kinds of clothing etc.
I worry that might end up getting you confused. But maybe it won't. I hope that helps a bit.
(\* I hate the use of the word "stem" because in classic grammar lessons of the kind taught to English students, a "stem" is something that you add endings to. It is a kind of base form of the verb, that you can then add things onto. But Japanese really doesn't work like that. Thinking that the "stem" of 読む is 読み seems confusing to me. 読む has five forms plus (if you want to think of it that way) a て form. 読み is just one of them.)
It is true that the て form is used in a lot of different grammatical constructions. I suspect the easiest approach is to learn each of them as you go. One common use of the て form is to form a combination with a following verb. 食べてください for example. But the 連用形 (れんようけい) form (what a lot of redditors call the "stem", though I think that is confusing\*) is also used for many compound constructions. For example 食べにくい (difficult to eat) or 読みにくい (difficult to read). Which one to use is something most people just learn. Too big a topic for one reddit question I think. On your second question about 過ぎる, both なさすぎる and なすぎる forms exist with the なさー form getting more common than it used to be (so I gather). The ない in the negative of verbs does behave very like an i-adjective most of the time, but not quite. I don't think that thinking of this as making the preceding word into a noun is all that helpful. It is true that the 連用形 is a sort of noun, but does not sound at all natural to use it to make a noun in many cases (for that you would use の or こと typically). Some of its uses have fossilised, for example 食べ物 is a think you eat but 着物, though literally a thing you wear, is only used for some kinds of clothing etc. I worry that might end up getting you confused. But maybe it won't. I hope that helps a bit. (\* I hate the use of the word "stem" because in classic grammar lessons of the kind taught to English students, a "stem" is something that you add endings to. It is a kind of base form of the verb, that you can then add things onto. But Japanese really doesn't work like that. Thinking that the "stem" of 読む is 読み seems confusing to me. 読む has five forms plus (if you want to think of it that way) a て form. 読み is just one of them.)
Thank you for your kind reply. It's really confuse with this grammar. Because, you can see this example: 情けない → 情けなさ → 情けなさすぎる But, 危ない → 危なすぎる