I found a Tumblr post recently about how roughly 300 words in any language is enough to talk about everyday things. There's a list of the 269 words/concepts here: http://funwithlanguages.tumblr.com/post/105572981908. My plan is to post every day finding and explaining a translation for one item of that list.

I don't know a lot of Japanese - I can read and write the kana, but little more than that. So if I get anything wrong, please correct me.

I also don't know how well the items in that list work in Japanese, but I'll do my best.

1. "be", or state of being

This is called the copula and it is not an actual verb in Japanese. There are two forms: です (desu) and だ (da), the polite and plain forms respectively. So to indicate that something is, you append one of those on the end. So to say "it is a dog" you would say:

「犬だ」 (inu da) (informal)
「犬です」 (inu desu) (polite)

The negative forms of だ and です are じゃない (janai) and じゃないです (janai desu). So to say "it is not a dog":

「犬じゃない」 (inu janai) (informal)
「犬じゃないです」 (inu janai desu) (polite)

Those are the non-past (present and future) forms of the copula. The past forms - "it was a dog" - are as follows:

「犬だった」 (inu datta) (informal)
「犬でした」 (inu deshita) (polite)

And the past negative - "it wasn't a dog":

「犬じゃなかった」 (inu ja nakatta) (informal)
「犬じゃなかったです」(inu ja nakatta desu) (polite)

So in conclusion, and for reference:

English:
is - was - isn't - wasn't.

Japanese:
だ　だった　じゃない　じゃなかった
da - datta - janai - janakatta

Polite Japanese:
です　でした　じゃないです　じゃなかったです
desu - deshita - janaidesu - janakattadesu

(1/269 concepts covered)