Thank you very much.I want to know“over joyed”＝“very happy”or is higner level or is right vocabulary in now ？Because I'm just started learning English recently,so I don't know how to study English grammar.

Overjoyed is a similar word to very happy.

Thank you!So I know new a word mow.

Thank you!But I want to know Why is [the second day]?And delete the vocabulary"at" ,add [studying with].

English very often requires the article. "____ is/are [article] [noun]." is a common setup and requires the article (unless it's a name).

For your second question: you are combining two sentences--
Today is the second day. I am studying with this app. --> Today is the second day THAT I am studying with this app. However, usually "that" is dropped in this sentence. "At" requires a physical location--I am at the university, at the bar. It can be used for "at 9 o'clock" but not for "day"

l understand now,thank you very much.How to study English grammar?I think is very difficult to study and understand it.And you speak is easy to understand with me.

You need a good grammar textbook...and read, read, read.