は wa and が ga, the Noun Markers
は wa and が ga cause much confusion among beginners, but their relationship can be summed up by replacing は with the phrase "on the topic of" to distinguish it from が. Note that in actual use, the "regarding the", "on the topic of" or "as for the" is not usually written since it is not normal English style. This format is only used to better show the main difference between は and が in this lesson. (Additional note: は is normally 'ha' on the hiragana chart, but when used as a particle, it is pronounced 'wa'.)
Example:
「ねこがたべています。」 (Neko ga tabete imasu.) translates literally as "The cat is eating."
「ねこはたべています。」 (Neko wa tabete imasu.) translates as "Regarding the cat, (it) is eating."
は is generally more flexible, because the "it" can be assumed, and is therefore recommended to novices who have not grasped the difference between the two. が specifically requires a verb to modify it, and if used incorrectly, can make the speaker seem silly.
Nevertheless, use of は is similar to changing the subject in a conversation, and foreigners are easily recognized because they change topics every sentence. While the relationship may seem complicated, once grasped it is much easier to understand.
Another way to look at this is が introduces a new subject, and は can be used to refer to that subject which which was mentioned before.
Example:
「ねこがいます。」(Neko ga imasu.) translates as "There is a cat."
「ねこはたべています。」(Neko wa tabete imasu) "Regarding the cat (that was mentioned before), (it) is eating."
So, when が is used, the thing in front of it is being emphasized, and when は is used, the emphasis is on what follows it.
は also has the specialized function of being used for comparisons as well.
Example:
「ねこがいます。」(Neko ga imasu.) translates as "There is a cat."
「いぬがいます。」(Inu ga imasu.) "There is a dog."
「ねこはたべていますけど、いぬはたべていません。」(Neko ha tabete imasu kedo, Inu ha tabete imasen) "The cat is eating, but the dog is not." or you can think of this as "As for the cat it is eating, but as for the dog, it is not."