Ist das richtig:

Das ist interessant für uns beide.

It's interesting for both of us?

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.

今天我学得很认真

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:

他给我们快乐地唱了一个歌儿。

She sang a song happily. If I use 得 it would mean that she ALWAYS sing happily.

Am I right?

I've been learning all the rules and cases of usage 了 (le) in Chinese (perfect aspect, modal participle and different structures) but I don't feel confident when using it when talking about past actions. Please, help me to understand how to use that correctly!

I know that the rule says that if we did something regularly in the past, then we don't need to put 了 in a sentence.

我去年每天去公园锻炼。

But when it comes to me that I'm going to tell a story (for example, about a particle day but in the past) I'm not sure how to use 了. For example:

昨天我去大学（了）。 I go to university almost every day, it's a habitual action but do I need to use 了 here because I'm speaking about the particular day and the moment of my life?

The second question is how many 了 should I use when talking about the past in every sentence? I mean, is it necessary to put it in all the sentences?

昨天我去大学（了）。然后回来（了）我家。然后我吃（了）饭。然后我去商店买书（了）。

Please, help me! I feel so mixed up with that grammar. Thank you in advance..