This particle has three main functions in Japanese. The first of these is to act as topic marker for conversations or texts. The main difference between a topic and a subject is that you can leave off a topic from a sentence, and it will still make perfect sense. This in contrast to a subject, which if removed creates a sentence with a gap in it:

Lets have a look at an example

きょうは　だれが　きましたか　Who came [here] today?

The same sentence on its own without the topic works just fine:

だれが　きましたか。Who came [here]?

However, the same sentence on its own without the subject won’t make sense.

きょう　はきましたか。 …came [here] today?

The second function of は is as contrast marker, as in an “[A]は something, [B]はsomething else” pattern.

Here is an example:

ちがいますよ。　これは　すしで、それはさしみ – [That's] not [quite] right. This is Sushi, and that’s Sashimi
Here a contrast is given between “this” and “that”, regardless of whether they were already topics.

The third function that は is the negative imperative when it follows a て form.

Here is an example;

きょう　こなくて　は　いけません　－　You have to come today.

In this use, it creates a negative imperative。The は could also be followed by だめ.

きょう　は　だめ 　－　Today is no good.

In our next post we will look at more particles.