The historical fiction piece I'm working on sounds modern with the use of contractions. I maybe skewed in my thinking but the shows I watch from that time period seem to avoid the use of contractions. I may have dug myself a deep hole, when I went back through all 86k words and opened about 20% of them. It is for sure you can't open them all, but if you contract them all, it sounds modern. I hope it is ok to post this little snip: “Something is coming, and we may have to move again,” he mumbled. His response was not what I expected, but I knew what it meant. I asked, “What are you up to? We are just getting our lives back and you tell me this?” “I can’t tell you more right now. We will just have to hope for the best.” “Does Carlos know what you and your co-conspirators are doing?” “No, Andrée, and you cannot say anything to anyone.” I will purchase the magic wand now if you please.
I find this to be a style choice, more than a rule. I tend to write narration without using contractions, but will use them in dialogue. But that is my personal style. So use them as you see fit to. Unless all of your characters are proper all the time, in which case it would be odd to use them. Other than that, it is entirely up to your best judgment. Though from what I have learned they do help dialogue feel more 'natural', and less mechanical, making it feel more realistic. Good Luck.
Dialogue without contractions sounds stilted and unnatural, which is a common problem in historical manuscripts: people who lived in the past didn't sound stilted and unnatural. People have always used contractions, whether they lived in 1300 or 2018. Sure, they used different contractions from today, but unless you're writing your MS in Ye Olde English I don't see how that matters. Just avoid modern slang like, "We're just getting our lives back and you tell me this HASHTAG OMG!" and you'll be fine.
Interesting that you used both a contraction and a non-contracted form of the same two words. The conversational 'can't' slips past us, almost without a ripple. 'Cannot' is more emphatic and formal and calls more attention to itself. As @Tenderiser suggests, if you're going for a historical feel, it's more important to avoid any slangy terms and modern ways of framing a thought, rather than avoiding contractions. Try to stick to concepts that would be current at the time. (Even a verb like 'zoomed,' for example, probably wasn't in use during Medieval times, to denote the rapid passage of something.) However, I'm sure people in 'olden' times spoke a colloquial form of their languages as well as using a more formal approach in certain situations. Just like today, really.
Interesting topic. How do you feel about words like: wow, fuck, okay, or cursing: damn, shit, goddamn Someone flagged “wow” in a fantasy story I wrote, said it took them out. That really surprised me.
I know plenty of writers who refuse to write contractions, I know plenty of others who live on them. Contractions aren’t anything new so depending on the period you’re writing, I’d say to use what feels appropriate when appropriate to set the desired tone. ‘Something is coming, and we may have to move again.’ Really doesn’t sound very ominus. If that character is supposed to be concerned, I’d think you could get away with ‘Something’s coming, and we’re going to have to move.’