At first, in the start, at the start/beginning
1. At first, initially, originally, in the beginning
Usually "at first" is used to describe the following situation:
When it begins, the situation is such-and-such. Later it turns out to be a contrast to the beginning situation.

I would like to know:
- whether it is a MUST that the beginning situation has to be the contrast of the later situation. (eg At first I liked the girl. Now I still liked the girl. Is it grammatically correct?)
- if so, do other words such as "initially, originally, in the beginning" have this requirement?

2. In the start
Does such a phrase exist?

3. at the start/beginning
- is it true we MUST use it like the following structure?
at the start/beginning of something

- is it possible for me to use them in the sense of "at first"?
1. It is somewhat unusual not to make a contrast after "at first".
"At first I liked her; now I still like her." [not "liked"]
It sounds odd without sounding exactly incorrect, much as it would sound strange to say "Stand up!" to a standing person.
In the sense of "at first", yes, other expressions ("initially", etc.) would sound equally odd in that context.

2. "in the start" -- impossible.

3. The "of something" may be omitted if it is clear from context what is meant.
"at the start (of ...)" and "at the beginning (of ...)" can be used like "at first":

At the beginning of our relationship, I found her difficult to understand; now I have adjusted perfectly.

I met her several years ago. At the beginning, I found her difficult ... [NO "of something"--"relationship" is understood]
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Is "at the start/beginning" the same as "at first" which they must make a contrast?

And it was what I was told relating to "at the beginning (of something)", "in the beginning":
- in the beginning means "at first"
- at the beginning is usually used with "of something" to mean the point in time/place etc. that something starts.

To me, "at the beginning" and "in the beginning" seems to differ.
Any ideas?
"To start/begin with"
I think they mean "at the start" as well.
Can they use to mean "at first"?