‘desu’ sentences

Japanese sentences end with so-called predicates, which consist of a verb and other words. The predicates are one of the most important elements in Japanese sentences because subjects are frequently omitted from sentences but predicates are not. This is also the location where ‘desu’ takes the role. The ‘desu’ helps other words to form a predicate. The two important ‘other words’ are nouns and adjectives.

Simply put, ‘desu’ is placed after these words and form a predicate. Interestingly though, ‘desu’ is not always required to form a complete sentence. Let’s explore these predicates formed with 'desu'.


The first predicates are with nouns. In the sentence ‘watashi wa nihon-jin desu’ (I am Japanese.), the ‘desu’ helps the noun ‘nihon-jin’ to form a predicate. This sentence without the ‘desu’, ‘watashi wa nihon-jin’, is an incomplete sentence. The complete predicate is ‘nihon-jin desu’ and the ‘desu’ cannot be simply taken away.

This ‘desu’ is grammatically called copula and the usage is closely equivalent to ‘to be’ (am) in English. Copulas link two words and often mean ‘equal’. In this sentence, the ‘desu’ creates the meaning of ‘watashi = nihon-jin’, which is the same as the ‘am’ creates ‘I = Japanese’ in English. In addition to this, ‘desu’ creates a polite positive form of present and future tense of the predicate.


The second predicates are with adjectives. Similar to ‘watashi wa nihon-jin desu’, both ‘watashi wa nemu-i desu.’ (I am sleepy.) and ‘watashi wa genki desu.’ (I am fine.) end with a predicate formed by an adjective and the ‘desu’, where ‘nemu-i’ is an i-adjective and the ‘genki’ is a na-adjective. These ‘desu’ can mean ‘equal’ and are used to create the polite positive forms of the present and future tense of the adjective predicates, ‘nemu-i desu’ and ‘genki desu’. However, these two ‘desu’ are different from each other in the relation to the kinds of adjectives.

Fortunately, the ‘desu’ for na-adjectives is used in the same way as the one for nouns. However, the ‘desu’ for i-adjectives is used in a different way. This ‘desu’ should be considered as the word which merely creates a polite form because it is also used for the past tense as well as present and future without changing its form as shown below. Furthermore, when you simply take away this ‘desu’, the predicates become informal but can still be complete predicates.
  • nemu-i desu [ present and future positive (polite) ]
  • nemu-katta desu [ past positive (polite) ]
  • nemu-i [ present and future positive (informal) ]
  • nemu-katta [ past positive (informal) ]

After reading this article, you know 'desu' is used for nouns and adjectives. To understand Japanese sentences, knowing the usage of ‘desu’ is essential. Although you may know that ‘desu’ is equivalent to ‘to be’ in English, it is not always true. When you understand how and when 'desu' is used, it surely helps your understanding of Japanese sentences.

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