Desu です

Japanese sentence structure is not the same as English sentence structure. 

 English sentence structure:

I - like - Japanese 

Subject - Verb - Object

Japanese sentence structure:

I - Japanese - like

Subject - Object - Verb

The verb 'to be' in Japanese is 'desu'. (is, am, are)

N.B. 'desu' itself is actually not a verb.

わたしはねこです

watashi wa neko desu

I am a cat

Here, 'watashi'  (I) is the subject, 'wa' is the particle indicating the subject, 'neko' (cat) is the object, and 'desu' (to be) is the verb.

あれはペンです

are wa pen desu

That is a pen

In this example 'are' (that) is the subject, 'wa' is again the particle indicating the subject, 'pen' (pen) is the object, and 'desu' (to be) is the verb.