They aren �t really interchangeable. I use them this way:
I need your help with a task. (not in)
I need your help with this.
Can you help me with this please?
I need your help in deciding which looks the best. (more formal)
I need your help in distinguishing the difference.
I need your help to decide which looks the best.
I need your help with my decision.
I need your help in the kitchen (in a place)
Have you ever been there before?
Have you ever seen him before? Yes. I have seen him before./ No. I �ve never seen him before.
We can use these if there is more information:
I had seen him before. I recognised him straight away.
Did you speak to your teacher? Yes. I saw him before I went on holidays.