Japanese Lesson 1 - It is a cat.

The most basic formula of Japanese is:
1. _____(だ)。(BLANK da.) - The casual form (to use with friends and family).
2. _____(です)。(BLANK desu.) - The polite form (to use with superiors like your boss or teacher).

Da/Desu roughly translates to "it is, is, am or are." It can also just act like a verbal period like saying "the sentence is over." While it can also add a level of politeness, formality and proper speaking, it is not required to form a sentence.

Example 1
猫だ。
Neko da.
It is a cat.

日曜日です。
Nichiyoubi desu.
It is Sunday.

厚い。
Atsui.
It's hot.

Talking About Yourself
Many people will introduce themselves with desu.
天野けい子です。
Amano Keiko desu.
I am Keiko Amano.

It could also be translated as "It's Keiko Amano."
誰ですか。 天野けい子です。
Dare desu ka. Amano Keiko desu.
Who is that? It's Keiko Amano.

Many people talking about themselves will just use Da/Desu without anything else.
九歳です。
Kyuu sai desu.
Nine years old.
When someone says something like that, they are obviously talking about themselves. There is no need to say "I am nine years old," when they are obviously the subject.

When Da/Desu Means Absolutely Nothing
There are times when da/desu has absolutely nothing to do with the sentence and is only there for politeness/proper Japanese purposes.
けい子はどこですか。学校です。
Keiko wa doko desu ka. Gakkou desu.
Where is Keiko? School.

Gakkou (学校) is school. Obviously Keiko is not a school so desu has only been added for politeness purposes.

Exercises
Left-click your mouse and roll it over the blank space below the romaji to see the answer in English. Feel free to post your score/questions in the comments.
1. 犬だ。
Inu da.
It's a dog.
2. 花子です。
Hanako desu.
I'm Hanako. / It's Hanako.
3. 女だ。
Onna da.
It's a woman.
4. 土曜日です。
Doyoubi desu.
It's Saturday.
5. 十歳。
Juusai desu.
I am ten years old. / It is ten years old.
6. 赤いです。
Akai desu.
It is red.
7. 新しいだ。
Atarashii da.
It is new.
8. 田中君です。
Tanaka-kun desu.
I am Tanaka-kun. / It's Tanaka-kun.
9. 寒いです。
Samui desu.
I am cold. / It is cold.
10. 牛乳だ。
Gyuunyuu da.
It's milk.

Vocabulary
犬 (Inu) Dog
花子 (Hanako) A feminine Japanese given name
女 (Onna) Woman
土曜日 (Doyoubi) Saturday
十歳 (Juusai) Ten years
赤い (Akai) Adjective form of Aka (赤) meaning red.
新しい (Atarashii) New
田中 (Tanaka) A Japanese family name
君 (Kun) A Japanese honorific title usually given to a boy or young man.
寒い (Samui) Cold
牛乳 (Gyuunyuu) Milk