还2、已经、有时（候）、快、快要、正

已经 points at the clock. It means the action counts as finished by now, not later. 了 is the little word that slams the door and marks the job as done. Stack them and 我已经吃饭了 sounds complete. Drop the 了 and 我已经吃饭 sounds like you stopped mid-thought. Treat 已经...了 as a married couple for now. Natives split them up only in longer, more advanced sentences. That 了 is the [completion 了](/grammar/le1-completed-aspect-62) you'll meet again and again.

也 means 'same as you' — someone else is in the same boat. 还 means 'and on top of that', like a second bill landing on the table. “游客想去喝茶，还要买门票。” ── [《这还是茶馆儿吗？》](/read/is-this-still-a-teahouse#s12). The ticket is the extra straw, not a case of 'me too'. 还 also means 'still' with ongoing stuff: 他还在开会 means he's still in that meeting. Word order matters: 还 always sits before the verb, never after it.

Chinese time words park in front of the verb, and 有时候 is no different. Two safe spots: right after the subject, or at the very start of the sentence. 我有时候点外卖 and 有时候我点外卖 both work fine. The one place it can't go is the end, which is exactly where English 'sometimes' loves to sit. 有时 is the same word with a haircut, shorter and a bit more written.

Both point at something happening right now. 正 is the spotlight, 正在 is the everyday workhorse. Bare 正 sounds clipped in 我正吃饭, like a sentence that lost its tail. Give it a 呢: 我正吃饭呢. Or hang a moment on it: 我正吃饭的时候，电话响了. Honestly, 我在吃饭 covers most real conversations in Beijing or Taipei. Save 正在 for when you really want to hammer the 'right this second' feeling.

他跑得很快 is about speed. Nothing is about to happen there. 快下雨了 means rain is on the way, and that 了 is doing all the flipping. 快要下雨了 says the same thing, just a touch fuller and softer. Both 快 and 快要 refuse to name an exact time. For that job, 就要 takes over: 明天就要考试了.