When two people are in conversation, the speaker will only look at the listener from time to time, in order to make sure _that_ the listener pays attention to what he or she is saying. -------------------------------------------------------------------------------------------------- Would you please tell me what I should fill in the blank and why? I have corrected the sentence to add "are" as the 4th word, changed does pay to "pays", and changed speaking to "saying". In the blank, you should use "that". The reason is mainly because of how you phrased the sentence, using "make sure" and "does pay attention." This phrasing implies a more certain feeling that the listener is paying attention, so that is more appropriate and just sounds better. If and whether imply a conditional and uncertain statement. This site might help you: http://painintheenglish.com/case/4789

...to make sure... uses _that_ Examples: Make sure that the door is locked! Make sure that you call your mother. ...if / whether ... See whether the door is locked, please. See if you can guess what I'm thinking. I'll find out whether you called or mother or not. By the way, it's unusual to say "pays attention" -- it's more usual to say "is paying attention." And "speaking" is not correct; "saying" is needed here. Here's your sentence with choices for some of the phrases: {informal / typical / more formal}. When two people are {talking / having a conversation / in conversation}, the speaker will {look at the listener now and then / occasionally look at the listener, only look at the listener from time to time} [no comma] in order to {see if / check whether / make sure that} the listener is {paying attention. / paying attention to what is being said. / paying attention to what he or she is saying.}

When two people are in conversation, the speaker will only look at the listener from time to time, in order to make sure if the listener does pay attention to what he or she is speaking. I added "are" (the 4th word) and "if" (fill in the blank)