When we think of expletives in English, we usually think of four-letter, naughty words. Those words are called expletives because they are filling in an empty spot (and of course, the word expletive comes from Latin--expleo, explere, to fill). There are also expletives used when you are using the word "to be." Exempli gratia, when you translate est, you can translate it as "he/she/it is" or "there is."
Est pax in Caelo. There is peace in heaven. OR Peace is in Heaven.
Here is a question confirmation:
Who or what is the sentence about? "There" is an expletive. It is filling in an empty spot. it is filling in to make the English sound better to us. It is an empty word with no meaning in this case. The real answer is peace. peace is the subject.
What is peace doing? it is existing. It just "is."
In Caelo is showing where it is existing. In is the preposition and Caelo is the noun object of the preposition (in the ablative because in here takes the ablative). Since that answers an adverb question (Where?,) it is adverbial and will be placed under the verb in a diagram.
After you or your student translate the expletive there, you could put parentheses around it to denote it is not there in the Latin. I do that when I am translating Authentic Latin.
If you have exercises or concepts you would like me to blog about, please let me know! I want to help you!