gramma--use of that

[amagarwa]

Can you please tell me which of the following is more appropriate

I believe that I can do xyx

or

I believe I can do XYZ


When should one use that?

thanks

[esledge]

I believe that I can do xyx

or

I believe I can do XYZ

In conversation (and a certain pop song comes to mind), you have probably heard both.

If the main verb of the sentence were a "bossy" verb, such as require or propose, we would need the "that."
Examples with "that":
The company requires that suppliers send packages via overnight mail.
I propose that you insure the package just to be safe.

In these cases, you need "that" for clarity! Without it, it is unclear what the role of the following noun should be.
Examples missing "that":
The company requires suppliers send packages via overnight mail.--> Does the company require suppliers? (suppliers as object of require) Or do suppliers send packages? (suppliers as subject of send)
I propose you insure the package just to be safe.--> Do I propose you? (you as object of propose) Or do you insure the package? (you as subject of insure)

Getting back to your example, I think the "that" is optional. "Believe" is not a bossy verb, demanding the Subjunctive Mood. Also, "I believe I" is not a likely confusion to arise from the absence of "that." The GMAT would probably prefer "I believe that I can XYZ," but wouldn't make the presence or absence of "that" the sole difference between two choices.