You may use "desu" (pronounced just "des") if you want to tell somebody something about a subject or about yourself. An example may be "My major is psychology" or "It is 9 o'clock"
You can translate です as "it is..."
しんりがく です (My major) is psychology.
Note: In Japanese you can often leave out the subject if it is obvious.
くじ です (It) is 9 o'clock.
アメリカじん です (I) am American.
Note: "desu" can also be translated as "am."
I'm in my first semester of studying Japanese as a college student and am by no means a master. I'm really just a beginner.
So please let me know if you think anything in this card is misleading or simply left out!
There is a lot of great content being put up on this site to help people with Japanese phrases and vocabulary but I thought we could use some grammar in there too! Let me know if you found this helpful and I'll try to put up more.
Also try leaving a simple comment in Japanese using です! Maybe tell us your name or where you're from or what you do. Practice makes perfect!