為什麼at first放開頭不加逗點 但in the end要呢？

At first, he seemed surprised by my question.
I had some difficulty at first recalling why we were there.
at first = initially

當副詞性質的「述部修飾語」放到句子的主詞之前時，通常會加上逗號，表示修飾語「越位」（如上面的第一句）。但在該子句較短時，有時不加這個逗號（請比較下面第一、二句的後面子句）。

I toyed with the idea of calling the police, but in the end I didn't.
Benny thought the president was sincere and sensitive, but, in the end, that's what counts.

In the end , the adv particle/phrase is used with the verbs in making obvious meanings clearer.

In the end, has the train arrived?
In the end, has she came back?
In the end, is the goods safely in?

In the first place(phrase) in making a list as a beginning, coming before all others in time or order
eg:-At first the problem seemed easy in the office.
eg:-At first the first page of the book is lost(=pt,.pp of lose)
eg:-At first we have to make new year resolution in the first month of the year.
eg:-At first the man who arrived is the assisstant.
eg:-At first the basic principles is burned at the riot.