Style guidelines and when to ask your instructor

 with thanks to the PTSA discussion board at my son's school

MS Word's style section (Go to File/Options , depending on the version) allows you to auto-magically select one or two spaces after a period. The key is consistency. The other key is to ask the instructor. Another area of variation is the Oxford comma: a, b, and c.  Still another stylistic area is the length of phrase that requires coordinating conjunctions to have a comma.  If you use a style guideline (APA, MLA, Chicago, IEEE, or who knows what else), the guideline will likely direct you on how whether one or two spaces follow a period.

This has a helpful article with screenshots on how to tick off (check off) the style settings in MS Word, which now defaults to one space after a period.

https://www.howtogeek.com/668309/how-to-stop-microsoft-word-from-marking-two-spaces-after-a-period-as-an-error/

More discussions and responses to those discussions here:

https://slate.com/technology/2011/01/two-spaces-after-a-period-why-you-should-never-ever-do-it.html and https://www.cultofpedagogy.com/two-spaces-after-period/ and https://archive.ph/yP0rz and http://theworldsgreatestbook.com/how-many-spaces-after-a-period/ 

https://www.chicagomanualofstyle.org/qanda/data/faq/topics/OneSpaceorTwo.html 

https://link.springer.com/article/10.3758%2Fs13414-018-1527-6  and https://www.theatlantic.com/science/archive/2018/05/two-spaces-after-a-period/559304/