Probably the most common grammar in the Japanese language. A Copula links a subject with another concept to describe it. In English this is done with a verb (to be) in the form of "is", "am", or "are".
For example, in English, Tom is well." uses is (the copula) to equate Tom with well. Although "to be" is a verb, there is no action that Tom is taking, but instead Tom is equated with well. Think of a copula like an equal sign. "Tom = well".
In Japanese, the copula is expressed, not with a verb, but by following an idea with だ, which links that idea with the subject (often an implied subject), and equates them.
Notice the "equation" follows a different order from English. Instead of "Tom = well", it's like "Tom well =". Sentence order in Japanese tends to put the information first, and the grammar following after as we will continue to see in the following sections.
Often the subject being equated or described is dropped and left implied. In this case, だ is still equating the preceding word with the subject; the subject is just invisible.
Despite how bare bones these sentences look compared to the English equivalents, these are indeed full Japanese sentences; the simplest sentences that the grammar allows. More information is left out compared to English, but is usually inferred from context.
Japanese doesn't have articles like 'the', 'a' or 'an'.
The subject (it/I/we/they) is often left implied. Japanese doesn't always require explicit subjects like English does, and words English would replace with "it" are simply omited. Japanese relies a lot more on context.
Apple and student can be singular or plural depending on context, there is no distinction in Japanese.
だ can even be dropped in very casual settings.