I don't know

1) used for saying that you are not certain about something

I don't know whether:

I don't know whether economic issues were discussed specifically.

2) used for saying that you do not completely agree

"It'll be boring." "Oh I don't know. It might be fun."

I don't know that:

I don't know that we need to discuss this.

3) used when you are giving your opinion and want to find out what someone else thinks

I don't know about you, but I'm starving.

4) used for criticizing someone

I don't know what/how/why etc:

I don't know what they were thinking of.

I don't know how he could be so cruel.