What is the difference between 初めは and 初めに？
I see that both mean "at first," but it seems like the phrases are not interchangeable.
Thank you for your explanation!
As far as I know, they have different meanings:
初めは At first.
初めに First / To begin with.
初めは is used to show how the state was in the beginning (usually because now the state is different)
初めは彼が好きじゃなかった。 At first, I didn't like him (maybe we are friends now, maybe not).
初めに marks a specific point in time (the beginning) where an action (as opposed to a state) happens.
彼らは５月の初めに大阪に着いた。They arrived in Osaka at the beginning of May.
I also think that 初めに can be used to show the first of a series of arguments that support each other, in the sense of "to begin with". For example, if my girlfriend asks me to buy an expensive, horrible dress for her, I could answer:
初めに、お金がない。To begin with, I have no money (and in addition, the dress is horrible).
Please provide more context or example sentences you tried to make with these words so we can help you with a specific problem. There might be other uses of 初めは and 初めに so if you are specific it's easier to help.