Verb-Desu

Desu to be; is; equals

..................................Affirmative...................Negative
Present....plain...............da...........................dewa nai
................polite..........desu.........................dewa arimasen
.
Past........plain...........datta.............................dewa nakatta
...............polite..........deshita..........dewa arimasen deshita

Kore wa hon desu. This is a book.
Akiyama-san wa sensei desu. Mr. Akiyama is a teacher.
Otenki ga ii desu. The weather is good.
Ano hito wa, kinô byôki deshita. He was sick yesterday.

Uses of WA

1. To show contrast: Sono hon wa yomimashita. I read that book (a specific book in contrast to any book) Note that hon (book) is not the subject the sentence, and the subject "I" is left out. Book is the topic of the sentence.
2.Unchanging facts. Akito-kun wa nihonjin desu. Akito is Japanese. The fact that Akito is Japanese does not change.
3.Place of specific person or thing. Okaasan wa amerika ni imasu. Mother is in America. Okaasan (mother) is a specific person.
4. Negative sentences. Okaasan wa nippon ni imasu. Mother is not in Japan.
5. Partial negative sentences Zenbu wa tabemasen deshita. I didn't eat them all.
6. "At least" or "At most" Ichi-jikan wa machimasen. I waited for at least one hour.
Hyaku en wa shimasen deshita. It didn't cost ¥100 at most.
7. Repeating action Sono ko wa "mama"-to itte wa nakijakurimashita. The child sobbed Mama over and over.