This lesson deals with past and present verb tenses.

We have already studied the present and future tense for positive and negative verbs. As a refresher:

Paati ni ikimasu.
Paati ni ikimasen.

Koora o nomimasu.
Koora o nomimasen.

Niku o tabemasu.
Niku o tabemasen.

I will go to the party.
I won't go to the party.

I drink cola.
I don't drink cola.

I eat meat.
I don't eat meat.

Note that desu is an exception to this because the negaive of desu is dewa arimasen or janai desu.

To change from present positive to past positive, take off the masu ending and replace it with mashita. To change from a negative present to a negative past, use the masen ending and add deshita to the end.

Kouen ni ikimashita.
Kouen ni ikimasen deshita.

I went to the park.
I didn't go to the park.

Omizu o nomimashita.
Omizu o nomimasen deshita.

I drank water.
I didn't drink water.

Niku o tabemashita.
Niku o tabemasen deshita.

I ate meat.
I didn't eat meat.

Note that again desu is an exception. You cannot say janai deshita. This rule will be explained later. 

Note also the word omizu. Mizu means water. The o is only added to be respectful. Many words often have this 'honorific' o added to the front.

-masu
-masen
-mashita
-masen deshita

desu
dewa arimasen
deshita
dewa arimasen deshita

ikimasu
nomimasu
tabemasu

paati
koora
niku
kouen
mizu
hon

positive present/future verb tense
negative present/future verb tense
positive past verb tense
negative pase verb tense

is
isn't
was
wasn't

to go
to drink
to eat

party
cola/soda
meat
park
water
book