This lesson will focus on the "i" type of adjectives.
Here are some "i" adjectives:
ookii (big)
chisaii (small)
akai (red)
samui (cold)
atsui (hot)
tsumetai (cold used for things like water)

IMPORTANT NOTE: The word "kirei" is the word "pretty" and it ends in "i" . However this is not a "i" type adjective, it is an exception so learn this right away, it is a "noun type" and I will explain more about this in the next lesson.

Here is how to use "i" type.
This is a big apple.
kore wa ookii ringo desu.

kore (this), wa (subject marker), ookii (big), ringo (apple), desu  (is).

Now to say  "this is a big red apple." you put "ookii" and "akai" together but you can not just put them together. There is a rule. You have to take the last "i" off "ookii" or the "i" on all other "i" type adjectives and put in "kute" like this:
"ookikute"
So then the sentence will become:
kore wa ookikute akai ringo desu.
 

You can also say:
This apple is big and red.
kono ringo wa ookikute akai desu.
 

To negate the adjective you take the "i" away and add "kunai".
Example:
Vocab: "hon" is "book".
First a simple example sentence
This book is expensive.
kono hon wa takai desu.

so to  say:
This book is not expensive.
you say:
kono hon wa takakunai desu.

Or you can say:
This is a non expensive book.
kore wa takakunai hon desu.
 

For the past tense version of the adjective you take the "i"away and add "katta".
Example:
Vocab "kinou" "yesterday".
Yesterday was cold.
kinou wa samukatta desu.
 

then to say the past "was not" you take away "i" from the negative form and add "nakatta".

Example
Yesterday was not cold.
kinou wa samukunakatta desu.
 

This is just a basic explanation of this concept and it is a very difficult part of Japanese to learn. So practice these by changing the tenses of these adjectives and you will do fine. In the next lesson I will teach the noun types which are easier and they work in a way you probably already know.