Being a Japanese word is even more demanding than being a Japanese worker. As long as workers obey their superiors they will usually manage to keep their job regardless of how well they perform; but words that don't contribute something valuable to a sentence are simply fired on the spot and thrown out the door, never to be heard from again.
Subjects and objects are the first and most obvious victims of this ruthless boss mentality, but practically any word can be dropped from any sentence if it is considered to be understood from the context or otherwise not worth the trouble of saying or writing.
It can even happen with verbs – which are often described as the one indispensable element of any proper Japanese sentence. While they are the most resistant parts of speech to getting dismissed for underpeformance, it turns out they aren't immune to it, either.
Let's look at one typical scenario where verbs are dropped. The following sentence appears in Reajer book 17, which features the text My Very Own Self by Taneda Santoka (read the free samples on Amazon and iBooks).
芭蕉《ばしょう》は芭蕉《ばしょう》、良寛《りょうかん》は良寛《 りょうかん 》である
At first glance there's nothing unusual with this sentence: it obviously means "Bashō is Bashō, Ryōkan is Ryōkan". But actually, there were supposed to be two verbs here, not one. The underlying structure, which is left implicit, is 芭蕉は芭蕉であり、良寛は良寛である. But the first verb has been dropped and the final verb である (in this case, a copula) now does double duty for both clauses.
Why was the first である dropped? Because it's identical to the final verb and can therefore be understood from it. Keeping it in the sentence would have created a repetition without adding anything valuable to the meaning.
The shortened sentence is similar to saying: "A is B, and X, Y" in English: the copula "is" covers both equations simultaneously, and there is no need to repeat it by saying "and X is Y". But this is fairly rare in English, as it can lead to ambiguities or simply sound like an incomplete sentence.
In Japanese, on the other hand, this kind of dropping can be done much more freely and not only in obvious contexts. If you feel like something important is missing in a Japanese sentence, it likely is missing – although native speakers won't miss it at all.
Liked this post? Subscribe to the free newsletter and get updates on new posts and ebooks!