A case of optional “that”: “check the” vs. “check that the”

Solution 1:

From what I've read online regarding this, the easiest thing for me to determine when to use "that" is to use it if everything after "that" can be used as a complete sentence.

Please check that the light is off.

"The light is off." is a complete sentence. "that" is needed.

Please check the light.

"The light" is not a sentence. Don't use "that".

Please check that your username and password are correct.

"Your username and password are correct." is a complete sentence. "that" is needed.

Please check your username and password.

"Your username and password" is a fragment. "that" is not needed.

Solution 2:

You are correct in the desire to use that in your sentence.

A great way to tell if your sentence is correct is to dissect it and see how it sounds. This is a huge help in forming sentences that may sound strange, but are correct.

For example, if we take your sentence and split it into two, we can better see how it is structured.

"Please check the username and password are correct."

Please check what?

• The username is correct.
• The password is correct.

"Please check that the username and password are correct."

Please check what?

• That the username is correct.
• That the password is correct.

When splitting the first sentence, the structure changes from the command of "Please check its correctness." to "It is correct." (Can't think of a cleaner explanation here)

Here is another example in a different light in case that didn't help clarify some:

"She and I buy groceries."

Who buys groceries?

• "She buys groceries."
• "I buy groceries."

"Her and I buy groceries."

Who buys groceries?

• "Her buys groceries." (This is why Her and I don't work)
• "I buy groceries."