わたしは教師(きょうし)です。
I'm a teacher.
過去形
わたしは教師でした。
I was a teacher.
未来形
わたしは教師になっているでしょう。
I will be a teacher (one day).
です is a linkin verb and changes according to the tense.
is - was - will be
です - でした - いるでしょう
It's simple, isn't it?
Details
わたし(は) I -> subject
教師 a teacher -> complement
です am -> liking verb
「わたしは教師です」の「です」は、日本語では『助動詞』に分類されています。
です is an auxiliary verb called 'jodoshi', which has a function of a liking verb in English and concludes the fact that I am a teacher. If you omit です, they would understand it, although it sounds poetic.
Have a good day.