I wonder if it's okay to use these interchangeably:
- You need just accept it.
- You need to just accept it.
No, it's not okay to use these interchangeably. They both work, but it depends on what they precede.
Here, need is followed by a noun, therefore no to.
Here, need is followed by a verb, therefore a to is required.
Both in one sentence:
In your example, accept is a verb, therefore a simple sentence would look like this:
By inserting just, you can extend it to:
Or probably other forms, which should still be correct, but less common:
Either way, to is never ommited, as it was in the first, simplest example.
Need can either be a regular verb, or a modal verb. When need is used as a modal verb, it does not take to. The modal verb is used in current day English mainly in the negative:
but I believe that some people also still use need as a modal verb when it is followed by certain adverbs, which would justify the use of "You need just accept it." I also believe that this usage was much more common 100 years ago.
If need is used as a modal verb, it does not get conjugated. Googling "he/she/it need simply" yields quite a few instances of this construction.
In conversational American English, you should use an article or "to" following "need".
It isn't "wrong" to omit the article or "to", but it is rather uncommon. If you do it you're going to sound like a know-it-all, and risk coming across as very rude.
Update: Case in point, see the comments.
The negative form, "need not" or "needn't", is less pompous but still very formal:
A teacher might scold a student in this way.
Saying "need just _" is going to sound unusual and