desu です

desu corresponds to "is" as in the example sentences below

kore wa hon desu	this is a book
otenki ga waruii desu	the weather is bad
watashi-no otosan wa funanori deshita	my father was a sailor

Notice the past tense slipped into the last sentence. Desu isn't used in all tenses like most "normal" verbs but can be used as below

desu - to be		Positive	Negative
Present	Plain	da	dewa nai
	Polite	desu	dewa arimasen
Past	Plain	datta	dewa nakatta
	Polite	deshita	dewa arimasen deshita
Probable	Plain	darō	dewa nai darō
	Polite	deshō	dewa nai deshō

desu is also known as the Copula

aru ある

aru means is or exists for the location of inaminate objects

okane ga arimasuka?	have you got any money?
shinbun wa, teberu ni arimasu	the newspaper is on the table

aru - is, exists		Positive	Negative
Present	Plain	aru	nai
	Polite	arimasu	arimasen
Past	Plain	atta	nakatta
	Polite	arimashita	arimasen deshita
Probable	Plain	aru darō	nai darō
	Polite	aru deshō	nai deshō

iru いる

iru means is or exists for the location of people or animals

neko wa doko ni imasuka?	where is the cat?
kino wa London ni imashitaka?	Were you in London yesterday?

iru - is, exists		Positive	Negative
Present	Plain	iru	inai
	Polite	imasu	imasen
Past	Plain	itta	inakatta
	Polite	imashita	imasen deshita
Probable	Plain	iru darō	inai darō
	Polite	iru deshō	inai deshō

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