Up to now, you have seen a few sentences in Japanese. A lot goes into a sentence, but perhaps the most fundamental kind of sentence is "X is Y". In Japanese, though, just a bit more has to be considered. If you want to say "is", you have to think about politeness.
First off, what is a base? A base is a very important strategic location in strategy and war. We take this analogy to describe the starting point for all addition to a verb phrase of any kind in Japanese. From one of a few kinds of bases (starting points), everything in conjugation is built. To know how to make the most basic "is" phrases like "X is/am Y", "X is/are not Y", "X was Y", and so on, we need only two of these starting points.
So, what is a copula? All copula means is the "to be" verb. In Japanese, this is である. We won't see it now because it's not useful, but it will be soon for grammar reasons. However, you will hear it's contracted form だ. This is the plain form. Japanese has plain and polite speech. Polite speech is used to be polite to those who aren't close to you. So, you need to also know that だ is replaced with です in polite speech.
In this lesson, we will look at only だ and です. To use them, we will need only one base and that's the one they're already in. This base is simply called the しゅうしけい. If you just want to refer to it the basic form, that's fine too!
Japanese has two basic tenses: non-past and past. Non-past refers to the present or future depending on context. The non-past is just the base form. In this lesson, all examples will be in the non-past tense so that you get used to using だ & です. All example sentences will be "There is" or "It is" kind of "X is Y" sentences so all you need to say is Yだ・です. This allows us to avoid any grammar we haven't seen yet.
It('ll be/'s) tomorrow.
That is/it is a dog.
Ah, a cat!
Um, I'm Japanese.
Meaning Note: Conceptualizing the word "to be" in all its facets with だ・です will be bad. In harder sentences like "I like being a doctor", the correct way to say it would not necessarily have or require だ or です. Always pay attention to how Japanese phrases things. This is more important than just learning individual words and hoping it'll be correct once you do an appropriate rearranging.