Here we work with a common basic sentence structure:

Declarative statements: something is something else

Kore wa ... desu

Kore - this thing here, near me

wa - topic marker: whatever precedes this word is the topic under discussion

... - insert a noun

desu - 'is' (note: the 'u' is silent, or 'swallowed')

Example: Kore wa kasa desu - This is an umbrella

 

Notes

1. Generally in Japanese, the verb comes last

2. Nouns do not have singular / plural forms

3. Nouns do not have gender associations

4. Verbs do not change if the subject is plural versus singular

5. Verbs do change to show tense (present or past) and positive or negative

 

Declarative statements: something is not something else

Kore wa ... dewa arimasen

Kore - this thing here, near me

wa - topic marker: whatever precedes this word is the topic under discussion

... - insert a noun

dewa arimasen - 'is not' (note: 'masen' is a common negative ending)

Example: Kore wa hon dewa arimasen - This is not a book (maybe it's a term paper)

 

Declarative statements: something was something else

Kore wa ... deshita

Kore - this thing here, near me

wa - topic marker: whatever precedes this word is the topic under discussion

... - insert a noun

deshita - 'was' (note: 'shita' is a common past tense form)

Example: Kore wa kuruma deshita - This was a car (maybe now it's a wreck)

 

Declarative statements: something was not something else

Kore wa ... dewa arimasendeshita

Kore - this thing here, near me

wa - topic marker: whatever precedes this word is the topic under discussion

... - insert a noun

dewa arimasendeshita - 'was not' (note the combination of past negative)

Example: Kore wa hon dewa arimasendeshita - This was not a book (maybe it is now; kind of a stretch!)

 

Further notes
1. Strictly speaking, "desu", and its variations, is not really a verb
2. "desu" is a copula, a word that links two words together
3. Although we generally translate various forms of "desu" into various forms of "is", it might be helpful to think of it as indicating a property or attribute
4. For example, "kuruma wa akai desu" - "my car is red", or "Tanaka-san wa bengoshi desu" - Mr. Tanaka is a lawyer."
5. So the first "something" is a noun, while the "something else" can be a noun or an adjective