I've been revising this grammar topic for ages but I can't still use these particles confidently. I know the structures:
V + 得 + Adj. / Adj. + 地 + V
I've also read that when the action was completed or it's a repeatable one, we use 得. When we describe a single action (in the past, present or future), we use 地. But there are still some doubts about using these particles. For example:
今天我认真地学习了。- Today I studied very hard.
Am I right that there are no difference? 地 + 了 — single, but completed action, while 得 is used just to describe how that action was done.
One more example:
今天我在比赛看见了一个人。他很快地跑步。— I tell a story about going to the competition where I saw a runner who ran fast. I don't know if he ALWAYS rans fast but I describe with 地 what I saw during the competition.
虽然我爷爷六十岁，但是跑得很快。— In this case I evaluate how my gradpa runs IN GENERAL with 得.
And the last one:
She sang a song happily. If I use 得 it would mean that she ALWAYS sing happily.
Am I right?